Here’s what you need to know:Cars lined up at a drive-through testing site in Bismarck, N.D., in October.Credit…Bing Guan/Reuters

Covid-19 hospitalizations in the United States hit an all-time high of 61,964 on Tuesday, as the raging pandemic continued to shatter record after record and strain medical facilities.

The number of people hospitalized with the coronavirus, tallied by the Covid Tracking Project, has more than doubled since September, and now exceeds the peak reached early in the pandemic, when 59,940 hospitalized patients were reported on April 15. A second peak in the summer fell just short of matching that record, with 59,718 hospitalizations on July 23.

Those spikes in April and July lasted only a few days and quickly subsided, but as winter approaches experts do not expect that this time. New cases are setting records in much of the United States, and rates of hospitalizations and deaths are following them upward.

The United States surpassed 10 million known cases on Sunday, and is averaging more than 111,000 new cases a day, a record.

While the number of patients continues to climb, a shortage of nurses and other medical personnel is limiting the ability to add more hospital beds to care for them.

The critical staff shortage, especially in Western states that struggle to attract doctors and other medical workers even in the best of times, is causing growing alarm, and driving some places to take extraordinary measures.

Gov. Douglas J. Burgum of North Dakota, which has the worst infection and death rates per person in the country, announced on Monday that health care workers who have tested positive but have no symptoms could continue to work in hospitals and nursing homes under certain restrictions, including that they treat only Covid-19 patients.

The Centers for Disease Control and Prevention’s guidelines allow the use of asymptomatic personnel during severe staff shortages. Mr. Burgum said his state was about two or three weeks away from facing “severe constraints” in hospital capacity.

When cases spiked in New York in April and in the South over the summer, health care professionals flew in from elsewhere to help. But now, officials describe a kind of national gridlock.

“Everywhere is either hard hit or is watching their Covid numbers go up, and are expecting to get a lot of flu patients,” said Nancy Foster, a vice president of the American Hospital Association. “The ability of health care professionals to pick up and leave their hometowns is very limited.”

That has left hospitals jockeying to find extra staff.

In Utah, for example, Intermountain Health Care, which runs some 22 hospitals, just added another 200 nurses, including 30 imported from New York. But Dr. Mark Briesacher, the company’s chief executive, said they were hard pressed to find more, a comment echoed by the heads of three other major medical groups in the state.

“We are at the tipping point,” Dr. Briesacher told a news conference on Monday. “We are beyond our normal capacity of caring for patients who are the most sick.”

Along the southern tier of neighboring Idaho, smaller hospitals have been told they can no longer rely on transfers to Utah of their most critical patients, said Toni Lawson, the vice president for governmental relations at the 40-member Idaho Hospital Association.

Even before the pandemic, the small rural hospitals in many Western states often depended on traveling nurses. But with Wyoming, Montana, the Dakotas and Utah all facing the same problem, the going rate for an I.C.U. nurse in the region has doubled to $120 an hour, more than small hospitals can afford, she said.

That leaves the small hospitals having to transfer some of their sickest patients to larger medical centers — and those beds are getting scarce, too, she said. “I do not want to say it is a crisis yet,” Ms. Lawson said, but if the system reaches capacity, “you start making difficult decisions of who gets what care.”

Although advances in coronavirus treatment have shortened hospital stays, the sheer number of patients is swamping hospitals.

Medical officials said they could try to fit in more Covid patients by stopping all but the most dire emergency surgery, or by recalling retired medical workers, but ultimately the only solution is fewer infections.

“The numbers across the board are troubling, but to address the hospital side of it, you have to address the public health side of it,” said David Dillon of the Missouri Hospital Association.

“Experimental and epidemiological data support community masking to reduce the spread” of the virus, the C.D.C. said on Tuesday.Credit…Joel Saget/Agence France-Presse — Getty Images

Breaking from its tentative recommendations on mask use thus far, the Centers for Disease Control and Prevention said on Tuesday that using masks benefits wearers, which is a step beyond its previous declaration that said wearing masks would only protect those around them.

“Experimental and epidemiological data support community masking to reduce the spread” of the virus, the C.D.C. said in a document that details scientific evidence supporting mask use. “Individual benefit increases with increasing community mask use,” it said.

The unequivocal statements are a departure from the agency’s previous language, which suggested that “the latest science may convince” Americans to wear masks and that mask use could prevent an infected person from spreading the virus to others. “The main protection individuals gain from masking occurs when others in their communities also wear face coverings,” it said.

The agency also offered an economic argument, saying that increasing the proportion of people who wear masks by 15 percent could prevent the need for lockdowns and cut associated losses of up to $1 trillion, or about 5 percent of gross domestic product.

The new document listed several studies that “have confirmed the benefit of universal masking,” as well as some observational studies that have given evidence of its usefulness, including an example of two masked hair stylists who had been experiencing symptoms but did not transmit the virus to any of their 67 masked clients who were later contacted. The document also referred to a study of 124 Beijing households in which mask use significantly cut transmission of the virus, and an outbreak aboard the U.S.S. Theodore Roosevelt in which face coverings appeared to have reduced risk of infection by 70 percent.

Experts said they were thrilled to see the change in the C.D.C.’s stance, and particularly the emphasis on face coverings that protect wearers.

“That matters for public-health messaging, because we don’t have people yet who are completely convinced about the benefits of masking until they see the C.D.C. say that it also protects you and your family,” said Dr. Monica Gandhi, an infectious-disease expert at the University of California, San Francisco.

“I would encourage every American to adhere to masking guidelines now that we hear more clearly today that this will protect you and others,” Dr. Gandhi said. “We cannot afford more lockdowns, but we can do our part to stop Covid-19 transmission and disease.”

Students at the School of Classical Ballet & Dance in Des Moines, where mask-wearing is mandatory. In a reversal, Iowa’s governor, Kim Reynolds, announced on Tuesday a mask mandate for some social gatherings.Credit…Kathryn Gamble for The New York Times

Gov. Kim Reynolds of Iowa, who has so far resisted issuing strict mandates during the pandemic, announced new restrictions on Tuesday, including mask requirements for large public gatherings.

The new order, which goes into effect Tuesday at midnight, requires masks for indoor gatherings of more than 25 people and outdoor gatherings of more than 100 people.

Masks will also be required for customers and employees of certain businesses, including salons, barbershops and tattoo parlors. Spectators at high school and youth sporting events will limited to two per participant.

“Iowa is open for business, and we intend to keep it that way,” Governor Reynolds said at a news conference. “That’s why it’s time for these additional mitigation measures.”

Over the last few weeks, coronavirus cases and hospitalizations in the state have increased sharply, placing Iowa’s health care system at risk of reaching capacity, the governor said.

In the last week, there has been an average of 3,888 cases per day in Iowa, according to a New York Times database, a 200 percent increase from 14 days prior.

“Like so many states, we’ve reached a serious point of community spread, and we can no longer pinpoint any one age group or type of activity that’s driving it,” Governor Reynolds said.

The Iowa governor had previously called mask mandates “feel good” actions, and she has refused to issue a statewide directive and blocked municipalities from enforcing their own edicts. Iowa was one of only a few states that never imposed a full stay-at-home order, and it let restaurants, bars and hair salons reopen earlier than most places.

Alex M. Azar II, the secretary of health and human services, on Capitol Hill in October.Credit…Michael A. McCoy for The New York Times

Federal health officials on Tuesday provided the most detailed projections to date about when a wider swath of Americans may be able to begin to receive Covid-19 vaccines, once any is authorized, and treatments outside of an experimental setting, and about what that could look like.

People deemed highest priority — perhaps health care workers or nursing home residents — could be vaccinated in December, with more widespread vaccination to come in the first months of next year, the officials said, laying out an ambitious timeline that could well be thwarted by delays.

The forecasting follows two encouraging developments on Monday: positive clinical-trial results from a vaccine front-runner and an emergency authorization for a Covid-19 treatment. The vaccine candidate, made by Pfizer and BioNTech, was found in an early analysis to be more than 90 percent effective in preventing Covid-19, and Pfizer is expected to submit its data for review by the Food and Drug Administration once it has the necessary safety data next week. No coronavirus vaccine has yet been authorized by the U.S. federal government. The treatment, a monoclonal antibody developed by the drug maker Eli Lilly similar to the one President Trump had taken, is beginning to be shipped out to states this week.

In a series of media appearances on Tuesday, Alex M. Azar II, the secretary of health and human services, mapped out a potential timeline for distributing Pfizer’s vaccine, if it receives authorization as is widely hoped. In December, the focus would be on vaccinating the most vulnerable groups. Those groups have not yet been determined, but recommendations from experts have zeroed in on health care workers, first responders and older Americans in nursing homes and assisted-living facilities.

Dr. Anthony S. Fauci, the nation’s top infectious-disease expert, echoed that projection in another appearance on MSNBC on Tuesday, saying that by December, doses would be available “for people who are judged to be at the highest priority.” That’s consistent with remarks Dr. Fauci and other officials have made in recent weeks.

By the end of January, Mr. Azar said he expected to have enough vaccine for all health care workers and first responders. (He also added senior citizens to that list in one appearance.) And by the end of March or early April, he expected to have enough of not just Pfizer’s vaccine, but also other vaccines in development, for all Americans, he said. Interim findings are also expected this month from another front-runner in the vaccine race, Moderna Therapeutics.

“The particulars are going to depend on seeing that data, and just seeing: Where does it look most effective — which population is it most effective and safe in?” Mr. Azar said on CNBC.

The hopeful projections from health officials take for granted that there will be no major delays, which is far from guaranteed. Pfizer’s vaccine results must still be evaluated for safety over a longer time period, data which aren’t expected to be available until next week. The F.D.A. could take longer than expected to evaluate Pfizer’s results. Complications could arise in what is a complex manufacturing and distribution effort. And supply could be further constrained if other experimental vaccines don’t work as hoped.

Federal health officials have vowed that Americans will be able to receive Covid-19 vaccines and treatments free of charge. But some seniors will have to pay about $60 to get Eli Lilly’s coronavirus drug, which must be administered via an infusion, federal officials indicated in a phone briefing with reporters on Tuesday.

The seniors affected are those on Medicare who don’t have supplemental coverage, and at least for now, they’ll be required to pay a 20 percent coinsurance fee for the administration of the drug, Paul Mango, deputy chief of staff for policy at the Department Health and Human Services, told reporters. The drug itself will come at no cost.

That same situation will not apply when vaccines become available. “No one will have to pay any out-of-pocket expenses for vaccine administration,” Mr. Mango said.

Milan on Friday. A significant portion of the country, including the Lombardy region, was put under lockdown by the government last week.Credit…Alessandro Grassani for The New York Times

As a brutal second wave of the coronavirus pandemic tests the capacity of the Italian health care system, the government will seal off additional portions of the country on Wednesday, regional governors announced.

Unlike the first wave, which was concentrated in the country’s north, the virus is now raging across Italy, from the highly industrialized Milan area to the country’s more precarious south. In an effort to prevent a nationwide lockdown, the government is progressively sealing off and locking down hard-hit areas around the country by assessing their level of risk on a weekly basis.

Starting on Wednesday, movement will be strictly limited to and within the central regions of Abruzzo, Umbria and Tuscany, as well as Basilicata in the south and Liguria on the coast, local authorities announced. A significant portion of the country, including the Lombardy region, was put under lockdown by the government last week.

As of Nov. 9, the average number of new cases per day over seven days was 32,684, according to a New York Times database, a 92 percent increase from 14 days prior.

“Nearly all the Italian regions are highly affected,” said Giovanni Rezza, the director of the prevention department at the Health Ministry, adding that new restrictions were necessary in a situation that “keeps worsening.”

With more than half of its beds occupied by coronavirus patients, the San Luigi hospital near Turin, in the highly affected northern region of Piedmont, placed dozens of beds inside the hospital’s chapel and a conference room. Benches, the tabernacle and sacred icons were also moved out of the hospital chapel in Latina, near Rome, to make space for Covid-19 patients.

Pictures of nurses distributing oxygen tanks to patients queuing in their cars outside an emergency room in Naples as the hospital scrambled to find empty beds recalled images circulating in the early days of the outbreak, when Italy became the first European country to be hit hard by the coronavirus. Doctors’ warnings also felt familiar.

“If the influx of people and hospitalizations stays the same,” Antonio Voza, the emergency-room director at the Humanitas hospital in Milan, told the Italian newspaper Corriere della Sera, “we won’t be able to hold out for a week.”

Protests by regional presidents in the locked-down areas have unnerved the government.

“Do we get it or not that we are at war?” Pierpaolo Sileri, Italy’s deputy health minister, shouted on Italian television on Sunday. “We are fighting to save Italy.”

Last month, as the coronavirus made a resurgence, the government imposed a set of restrictions, including early closings for restaurants and bars, which health authorities said proved successful in flattening the virus’s reproduction rate.

But with more than 500 cases per 100,000 people, a nationwide lockdown seems increasingly likely, and some Italians are now wondering whether Italy learned any lessons from the devastating first wave.

Andrea Crisanti, a professor of microbiology at the University of Padua and a top scientific consultant on the virus in the Veneto region during the initial outbreak, said that the Italian government missed an opportunity to tame a second wave by failing to set up an efficient contact-tracing system and to sufficiently increase its testing capacity.

“We don’t have a plan to get out of this,” Mr. Crisanti said on Italian television. “We have hopes.”

Federal Medical Center in Fort Worth. Nearly three out of five inmates in Texas jails and prisons who died of the virus were eligible for parole at the time, researchers found.Credit…Larry W Smith/EPA, via Shutterstock

Inmates in Texas prisons and jails have tested positive and have died at substantially higher rates than the national average for prisons, and more than half of those who died were eligible for parole at the time, a University of Texas at Austin study found.

At least 187 Texas inmates and guards have died, and more than 41,000 have been infected by the coronavirus, according to a New York Times database. By contrast, the federal prison system, which is roughly comparable in size, has had 143 deaths and about 24,000 infections.

Of the nation’s 10 largest prison systems, only Florida reported a higher infection rate per 10,000 inmates than Texas, the largest system in the country, and Michigan and Ohio had higher death rates, as of Oct. 4, according to the study. The Texas system has also had by far the most deaths of employees.

One Texas prison — the Rufus H. Duncan Geriatric Facility near Diboll — has had nearly 6 percent of its inmates die of Covid-19, the report said. That prison has reported at least 339 infections.

The report, published this month by the Lyndon B. Johnson School of Public Affairs at the University of Texas at Austin, found that Texas has made much less progress in reducing coronavirus deaths in prison than have Michigan, Ohio and New Jersey, which all had high death tolls early in the pandemic.

Jeremy Desel, a spokesman for the prison system, said the report failed to properly credit the state for performing universal prison testing, a rarity among correctional systems.

“Most inmates have little to no symptoms and recover,” he said, “but tragically, some have succumbed to Covid-19.” Mr. Desel said most of those who died “were elderly and had numerous pre-existing conditions.”

Public school students in Philadelphia will continue virtual learning into the spring, officials said on Tuesday. Credit…Matt Rourke/Associated Press

The Philadelphia public school district has again put off plans to bring students back to classrooms for at least some in-person instruction. Now, as coronavirus cases multiply in the city and across the state, remote learning will continue “until further notice.”

“We have decide to remain 100 percent virtual at this time,” the superintendent, William R. Hite Jr., said Tuesday afternoon.

The district had tentatively planned to bring younger students back to classrooms for hybrid learning on Nov. 30. Students in third through 12th grade with complex needs were to return to classrooms in January, with high school and technical students to follow in February. All that is up in the air now.

“We hope to see these children in school before the spring, but it’s all going to be based on the advice from the health community,” Dr. Hite said.

Philadelphia is the latest major American public school district to postpone or retreat from in-person instruction.

Boston reversed course in late October, sending the few high-needs students who had come back to classrooms home again and postponing plans for young learners to re-enter classrooms. Chicago has no set date to reopen its classrooms. Across Maryland, districts are reconsidering or reversing plans.

San Francisco, amid widespread dysfunction, does not have a timeline for its schools to reopen, although coronavirus cases remain low in the city. San Diego has halted a phased return to classrooms until January. And only a few Los Angeles public school students are back in school buildings; most classrooms are unlikely to reopen before January.

While public-school students in the city still learn entirely online, around 95 independent, parochial and charter schools in Philadelphia have reopened for at least some in-person education, according to Dr. Thomas Farley, the city’s health commissioner.

“We’re not recommending that they shut down for in-person education,” Dr. Farley said of other schools in the city, although he noted that that might change if the city’s daily case counts continue to rise.

There have been cases and case clusters at several independent schools in Philadelphia, but only three appear to have had in-person transmission, Dr. Farley said. That tracks with a growing consensus among scientists that there is little clear evidence of significant coronavirus transmission in schools, especially among younger children.

The Philadelphia Federation of Teachers praised the decision to put off the resumption of in-person instruction. “Our voices and concerns have been heard,” Jerry Jordan, the president, wrote in a letter to members, according to The Philadelphia Inquirer. “This is a big victory for this union.”

“I know we are all tired, but the virus is not,” Mayor Ras J. Baraka of Newark said.Credit…Bryan Anselm for The New York Times

Officials in Newark announced on Tuesday a nightly curfew in three ZIP codes with high rates of positive coronavirus test results, effective at 9 p.m. The move comes a day after Gov. Philip D. Murphy of New Jersey ordered all restaurants and bars statewide to close for indoor dining at 10 p.m., starting Thursday.

A directive from Mayor Ras J. Baraka of Newark set out a series of new citywide restrictions: indoor and outdoor gatherings should not exceed 10 people, nursing homes must stop allowing family visits and all team sports must stop for 14 days. Players and coaches must be tested before practices can resume.

The state’s largest city, Newark, saw the positivity rate over three days last week climb to more than 19 percent.

“Stricter measures are required in the city’s hot spots in order to contain the virus and limit the spread,” Mayor Baraka said in a statement. “I know we are all tired, but the virus is not.”

It was unclear if the state would step in to restrict Newark’s ability to make its own pandemic-related rules; on Monday, the governor’s top lawyer, Parimal Garg, said that state law superseded municipal orders.

The affected Newark ZIP codes — 07104, 07105, and 07107 — include the Ironbound restaurant district, which continues to have the city’s highest rates of positive test results in spite of limits imposed two weeks ago. Residents in the three areas should remain off the street after 9 p.m. on weekdays and 10 p.m. on weekends, unless traveling to or from work, or in the case of an emergency.

In New York City, Mayor Bill de Blasio also expressed growing fears about the virus’s resurgence, saying on Tuesday that the seven-day average rate of positive test results citywide had increased to 2.31 percent in what he called a “warning sign” of a potential second wave of infections.

At the beginning of September, that figure had dropped below 1 percent. But over the last month, it has been steadily creeping upward, reflecting what city officials said was an increase in transmission within the community, as well as travel-related infections.

Health officials are watching cases rise in specific neighborhoods, including Tottenville in Staten Island where the positivity rate exceeded 6 percent the past seven days, according to city data. But the rising numbers could have major implications for all New Yorkers and threaten to bring back the devastation the region experienced in the spring.

Mr. de Blasio said on Tuesday that if the seven-day average rate crosses 3 percent public schools would be forced stop in-person instruction and move to remote learning. A continued increase could force the city to shut down some or all businesses again, he added. While hospitals in the city have so far been able to handle new patients, the rise in cases could put a strain on health care, he said, mirroring the difficulties that cities are experiencing nationwide.

“And of course the most horrible dilemma, the most horrible consequence: starting to lose lives on a large scale, particularly our elders,” Mr. de Blasio said. “This is our last chance to stop a second wave. If we aren’t able to stop it, there will clearly be lots of consequences that will remind us to much of where we were before.”

Statewide, the seven-day rolling average rate of positive test results was 2.23 percent, Gov. Andrew M. Cuomo of New York reported on Tuesday.

Children may carry antibodies effective against the new coronavirus following infections with related viruses, a new study suggests.Credit…Mark Abramson for The New York Times

It’s been a big puzzle of the pandemic: Why are children so much less likely than adults to become infected with the coronavirus and, if infected, less likely to become ill?

A possible reason may be that many children already have antibodies to other coronaviruses, according to researchers at the Francis Crick Institute in London. About one in five of the colds that plague children are caused by viruses in this family. Antibodies to those viruses may also block SARS-CoV-2, the new coronavirus causing the pandemic.

In a study published Friday in Science, the group, led by George Kassiotis, who leads the Retroviral Immunology Laboratory at the institute, reports that on average only 5 percent of adults had these antibodies, compared with 43 percent of children.

In March, as the pandemic was just beginning, Dr. Kassiotis and his colleagues decided to develop a highly sensitive antibody test. To assess it, they examined blood samples taken before the pandemic from over 300 adults and 48 children and adolescents, comparing them with samples from more than 170 people who had been infected with the new coronavirus.

The scientists expected samples taken before the pandemic to have no antibodies that attacked the new coronavirus. Those were to be the controls for the test the scientists were developing.

Instead, they found that many children, and some adults, carried one antibody in particular that could prevent coronaviruses, including the new one, from entering cells. This antibody attaches itself to a spike that pokes out of coronaviruses.

While the new coronavirus has a unique tip on its spike, the base is found in all coronaviruses, Dr. Kassiotis said. In lab tests, antibodies to the base of the spike prevented the new coronavirus from entering cells in order to reproduce.

Now the researchers are planning to expand their study to monitor thousands of children and adults. Some have antibodies that can block the new coronavirus in lab tests. Others do not.

The L.S.U. Tigers football team at the start of a game in October. Their game with Alabama was postponed after positive test results and contact-tracing rules left the team short-handed.Credit…Derick E. Hingle/USA Today Sports, via Reuters

The Southeastern Conference said Tuesday that it had been forced to postpone two more college football games this weekend, including the fabled Alabama-at-Louisiana-State matchup, because of the pandemic.

A game between Texas A&M and Tennessee was also delayed on Tuesday, one day after the league said it had postponed this weekend’s planned matchup between Auburn and Mississippi State.

“While it is unfortunate to have multiple postponements in the same week, we began the season with the understanding interruptions to the schedule were possible, and we have remained focused throughout the season on the health of everyone around our programs,” Greg Sankey, the league’s commissioner, said in a statement on Tuesday.

“We must remain vigilant, within our programs and in our communities, to prevent the spread of the virus and to manage activities that contribute to these interruptions,” he added.

Officials said the games were postponed because positive tests for the virus and contact-tracing rules left L.S.U., Mississippi State and Texas A&M short-handed once other injuries were considered.

The SEC, home to 10 of the last 15 national champions in football, is one of the most widely watched and closely scrutinized leagues in American sports. It got through the first several weeks of its delayed, conference-only season with no substantial problems, but the league began to encounter trouble in October, when it first had to postpone games.

Although L.S.U., which is 2-3 this season, has hardly lived up to the ordinary standard of a reigning national champion, its game against Alabama was widely anticipated and was to air in prime time. (President Trump attended last season’s game.)

It was not clear whether that game would be rescheduled, in part because L.S.U. is already penciled in to play at Florida on Dec. 12, the ordinary landing spot for postponed games. Texas A&M and Tennessee will play on that date, as will Auburn and Mississippi State.

In its statement on Tuesday, the league raised the possibility that it would reschedule some games for Dec. 19 — the day the conference title game is scheduled in Atlanta.

Nationwide, more than four dozen games involving teams in the Football Bowl Subdivision, college football’s most prominent and lucrative tier, have been postponed or canceled this season for virus-related reasons.

Restaurants, gyms, cafes and other crowded indoor venues accounted for some 8 in 10 new coronavirus infections in the early months of the U.S. epidemic, according to a news analysis.Credit…Taylor Glascock for The New York Times

Restaurants, gyms, cafes and other crowded indoor venues accounted for some 8 in 10 new coronavirus infections in the early months of the U.S. epidemic, according to a new analysis that could help officials around the world now considering curfews, partial lockdowns and other measures in response to renewed outbreaks.

The study, which used cellphone mobility data from 10 U.S. metro areas from March to May, also provides an explanation for why many low-income neighborhoods were hardest hit. The public venues in those communities were more crowded than in more affluent ones, and residents were more mobile on average, likely because of work demands, the authors said in the research published in the journal Nature on Tuesday. The metro areas were Atlanta, Chicago, Dallas, Houston, Los Angeles, Miami, New York, Philadelphia, San Francisco and Washington D.C.

Infectious disease models had provided similar estimates of the risk posed by crowded indoor spaces, going back to February; all such models are subject to uncertainties, due largely to unforeseen changes in community behavior. The new analysis provides more precise estimates for how much each kind of venue contributed to urban outbreaks, by tracking hourly movements and taking into account the reductions in mobility from lockdown restrictions or other changes that occurred during those first crucial months. It did not model infection in schools or office workplaces.

“Restaurants were by far the riskiest places, about four times riskier than gyms and coffee shops, followed by hotels” in terms of new infections, said Jure Leskovec, a computer scientist at Stanford University and senior author of the new report, in a conference call with reporters. The study was a collaboration between scientists at Stanford, Northwestern University, Microsoft Research and the Chan Zuckerberg Biohub.

Public officials across Europe and in parts of the United States, including Gov. Philip D. Murphy of New Jersey, have begun to institute partial closures of restaurants and bars, or limited indoor hours, as new infections have surged in recent weeks. In New York City, a spike in virus cases threatens the city’s recovery and could mean “a lot more restrictions,” Mayor Bill de Blasio said Monday.

These measures are especially important in lower income areas, the new study suggests. Infections exploded in many such communities last spring, and the new model provides one likely explanation: Local venues tend to be more crowded than elsewhere. Grocery stores, for example, typically have some 60 percent more people per square foot, on average, than in more affluent areas, and shoppers stay inside longer. And residents are less able to shelter at home.

“We think a big reason for that is that essential workers had to be on the job, they weren’t working from home,” said Serina Chang, a co-author also at Stanford.

By focusing on indoor public venues, the researchers could also model the impact of partial restrictions. Limiting restaurant occupancy to one-fifth of capacity, for example, would reduce new infections there by 80 percent, while preserving some 60 percent of customers.

“These are important tradeoffs,” Dr. Leskovec said. “Our work highlights that it does not have to be all or nothing,” when implementing restrictions.

Gov. Pete Ricketts of Nebraska said that he would quarantine for 14 days after someone he dined with tested positive. Cases and hospitalizations are rising in the state.Credit…Nati Harnik/Associated Press

Gov. Pete Ricketts of Nebraska said on Tuesday that he would go into quarantine after dining with someone who tested positive, just a day after announcing new measures to halt an alarming spike in virus cases and hospitalizations.

Mr. Ricketts’s office said in a statement that he and the state’s first lady, Susanne Shore, had dinner outdoors with three other people on Sunday night, and that one person from their party tested positive on Monday.

Since the virus takes a few days to incubate before its presence in the body is detectable, Gov. Ricketts said at a news conference Tuesday that his wife had gone on the website for Test Nebraska, the state’s free testing program, and had signed the couple up for Thursday.

He was already being tested weekly, he said, and got a negative result last Wednesday. Given the recent contact, though, he said the couple planned to stay in quarantine through Nov. 22 as a precaution.

The governor said he was “feeling great,” and that he had ridden the equivalent of 23 miles on his stationary bicycle earlier in the day.

The fact that the risk of exposure emerged from an outdoor dinner with a few friends at his private residence in Omaha demonstrated “that we all need to be careful with regard to social interactions,” Mr. Ricketts told reporters.

New cases and hospitalizations in Nebraska have been on a steep upward trajectory throughout the fall. The state tallied an average of 1,846 cases a day, or 95 for every 100,000 people, over the last seven days, an increase of 127 percent from the average two weeks earlier. Only the Dakotas, Iowa and Wisconsin recorded more cases per capita during that period.

By comparison, France, which has entered a second nationwide lockdown, averaged 73 cases a day per 100,000 people over the last week.

At a news conference on Monday, Mr. Ricketts announced new measures to halt the spread of the virus, including requiring masks in businesses where people are in close contact for more than 15 minutes. But restaurants, bars and places of worship remain open, albeit with six feet of distance required between parties. Indoor gatherings are limited to 25 percent of capacity. The new measures will go into effect on Wednesday through Nov. 30.

Some critics said the measures were too little, too late. Over the weekend, some Nebraska doctors had pleaded for help to lessen the pressure on hospitals.

The number of hospitalized #COVID patients in Nebraska is skyrocketing. Our community and our hospitals are suffering and our HCWs are so tired. We are not an unlimited resource. We need directed health measures now @GovRicketts pic.twitter.com/52VOgURUx1

— Dr. Angela Hewlett (@hewlett_angela) November 7, 2020

In response, Mr. Gage posted screenshots of some of the tweets and accused the doctors of targeting Mr. Ricketts and supporting Democrats.

Nearly 800 virus patients are hospitalized across the state, according to the Covid Tracking Project. The outbreak is concentrated along Nebraska’s eastern border with Iowa.

At least three governors have tested positive for the virus, along with a fast-growing list of other federal and local officials. The governors include Kevin Stitt of Oklahoma, Mike Parson of Missouri and Ralph Northam of Virginia. In August, Mike DeWine, the governor of Ohio, received a positive result from a rapid test before a scheduled appearance with President Trump. He tested negative hours later, using a more reliable P.C.R. test.

Other governors have had to quarantine after possible exposure to the virus, including Philip D. Murphy of New Jersey, Mark Gordon of Wyoming and Brian Kemp of Georgia.

Mr. Ricketts was scheduled to give a virtual news conference on Tuesday at 3 p.m. Eastern.

A technician at work in a Sinovac Biotech lab in Beijing, where the company is producing its CoronaVac vaccine.Credit…Kevin Frayer/Getty Images

Brazil said on Monday that it had halted a late-stage trial of a Chinese vaccine, which had been considered a global front-runner in the race to develop a protective shot for the coronavirus, after a “serious adverse” reaction in a participant.

The Brazilian health regulator provided little information on its decision, and did not say whether the reaction was related to the vaccine, called CoronaVac and produced by the Chinese company Sinovac, or coincidental.

The participant who had the reaction became ill on Oct. 29, according to the authorities. They did not divulge where in Brazil the vaccine had been administered or what had happened to the volunteer, citing patient confidentiality. In a statement, they said that such a “serious adverse incident” might include death, disability, hospitalization, birth defects or other “clinically significant events.”

CoronaVac is one of 11 experimental vaccines, produced by some of the world’s foremost pharmaceutical companies, currently in Phase 3 trials.

On the same day that Brazil suspended the Sinovac trial, the American company Pfizer announced that an early analysis of its coronavirus vaccine trials suggested that its drug was more than 90 percent effective in preventing the virus that causes Covid-19.

Sinovac’s drug was seen in China as a leading candidate. But in Beijing’s push to get a Chinese vaccine to be the first on the global market, officials stretched the definition of “emergency use.” They have permitted tens of thousands of people to receive the Sinovac vaccine and two other locally made vaccines, despite having not yet concluded Phase 3 trials.

Adverse effects are not unusual in Phase 3 trials. AstraZeneca and Johnson & Johnson both paused their trials after a few volunteers fell seriously ill, resuming them six weeks later, in October, after concluding that the illnesses were not related to the vaccines

The details of why the Brazilian health regulator had paused the trials were uncertain.

In a statement issued on Tuesday, Sinovac said Instituto Butantan, the medical center coordinating the Brazilian trials, had deemed the “serious event” not related to the vaccine. The company said it was “confident in the safety” of its vaccine.

According to news reports, the institute confirmed that a volunteer had died, but officials there said they were “surprised” by the government’s decision to halt the trials.

Dimas Covas, head of the institute, told a Brazilian television network that he found the government regulator’s decision strange “because it’s a death unrelated to the vaccine.”

Prof. Kim Mulholland, a pediatrician at the Murdoch Children’s Research Institute in Melbourne, Australia, called it “alarming” that Instituto Butantan appeared to have no idea why the trial had been stopped. “It leaves me wondering who had done that and why,” Professor Mulholland added. “That’s the question that really needs to be answered because this is a violation of the normal process.”

A mass grave being prepared for mink in a military area near Holstebro, Denmark, on Monday.Credit…Morten Stricker/Ritzau Scanpix, via Associated Press

Denmark’s plan to kill its farmed mink is on hold because of a question about the government’s legal authority to order the cull, the Danish environment and food minister, Mogens Jensen, said in an interview on Denmark’s TV2 on Tuesday.

Last week, Denmark announced that it would kill all its farmed mink because of coronavirus infections and virus mutations.

In Tuesday’s interview, Mr. Jensen said he was not aware that the government did not have legal authority to require all healthy mink to be culled.

Mink farmers are still encouraged to cull their mink and the government is seeking legal approval, Mr. Jensen said.

Farms with coronavirus infections and other farms within a radius of about five miles are required to cull their animals under a pre-existing order.

Denmark’s concern is that a mutated variant of the coronavirus that came from mink and that has infected some people could diminish the effectiveness of potential vaccines.

The World Health Organization has said there is no evidence yet that vaccines would be affected. And there is no evidence that the mutated virus, one among many existing mutations in both people and mink, is more transmissible or causes more serious illness than any other variant.

Saeb Erekat, a Palestinian leader and negotiator, in 2019.Credit…Mohamad Torokman/Reuters

JERUSALEM — Saeb Erekat, a senior Palestinian leader and negotiator who passionately advocated the establishment of an independent Palestinian state as a resolution to the Israeli-Palestinian conflict, has died. He was 65.

His death was confirmed by the hospital, Hadassah Medical Center in Jerusalem; his daughter and his Fatah party. The hospital said Mr. Erekat, who had previously had a lung transplant, was admitted in critical condition on Oct. 18 with Covid-19, the disease caused by the coronavirus. He required immediate ventilation and resuscitation. After his condition did not improve, died from multiple organ failure.

For three decades, as a close confidant of the Palestinian leader Yasir Arafat and his successor, President Mahmoud Abbas of the Palestinian Authority, Mr. Erekat was one of the most prominent voices of the Palestinian cause.

As the chief negotiator for the Palestinians, he was one of the main authors of key parts of the landmark Oslo peace accords of the 1990s, the first agreements between the Israelis and the Palestinians, which established Palestinian self-government in parts of the occupied West Bank and Gaza Strip.

Although his public statements sometimes gave him the image of a firebrand, he was liked and respected by many of his American and Israeli counterparts, who found the Western-educated diplomat frank and knowledgeable.

Mr. Erekat was initially treated at home when he became ill with the virus, but on Oct. 18, he was taken to Hadassah Medical Center in Jerusalem in an ambulance.

His life’s ambition of helping to bring about Palestinian statehood and an end to Israeli occupation eluded him, a source of frustration.

“I’m not finished with what I was going to do,” he recently messaged Tzipi Livni, the former Israeli foreign minister and one of his main negotiating partners. “I overcame a lung transplant, and I’ll defeat this Covid.”

GLOBAL ROUNDUP

Foreign Minister Peter Szijjarto of Hungary with Prime Minister Hun Sen of Cambodia in Phnom Penh last week.Credit…Agence France-Presse — Getty Images

Peter Szijjarto, the Hungarian foreign minister, came to Southeast Asia to promote trade and investment. He would leave possibly having infected at least five people in two countries with the coronavirus.

Last Tuesday, Mr. Szijjarto tested positive for the virus upon arrival in Thailand, which has kept its caseload low by defending its borders. The day before, he had met with Prime Minister Hun Sen of Cambodia and other senior leaders in a country that has limited its coronavirus cases to a few hundred people.

Mr. Szijjarto was asymptomatic, Thai health officials said, and he left Thailand by private jet soon after his positive test.

Fast-forward a week, and a Hungarian diplomat in Bangkok who met with Mr. Szijjarto was confirmed to have tested positive for the virus, according to Thai health authorities.

On Monday, three people in Cambodia who had contact with Mr. Szijjarto — a Cambodian lawmaker and agriculture ministry official, and the Hungarian ambassador to Vietnam and Cambodia — were found to have the virus, according to Cambodian health officials. Two days before, a Cambodian bodyguard who had protected the Hungarian foreign minister during his trip also tested positive.

Mr. Hun Sen, who met with Mr. Szijjarto without social distancing protocols, is now in home quarantine. Schools in Phnom Penh, the Cambodian capital, have shut again, after having opened last week after a long coronavirus closure, as a precaution following the Hungarian foreign minister’s visit.

In other developments around the world:

Australia’s prime minister, Scott Morrison, said on Tuesday that the country was considering opening its borders to several Asian countries, including Japan, South Korea, Singapore and Taiwan. Australia already has a one-way travel bubble with New Zealand, which allows visitors from that country to enter Australia, but not the other way around. “I think we proceed cautiously,” Mr. Morrison told reporters in Canberra.

Returning citizens of the Solomon Islands will be prosecuted if they lie on predeparture forms about their exposure to the coronavirus, Prime Minister Manasseh Sogavare said Monday. “My government will hold these people responsible for endangering our people and our country,” he said in his weekly address to the nation, the local news media reported. The Solomon Islands, which recorded its first coronavirus case last month, now has a total of 16, all of them detected among people in quarantine after arriving from overseas.

Restaurants and nightclubs in Moscow will be forbidden from serving customers between 11 p.m. and 6 a.m. for two months to reduce the spread of the virus, Reuters reported. Moscow’s mayor, Sergei Sobyanin, wrote on Tuesday on his website that these measures would be in place from Nov. 13 until Jan. 15. During that period, schoolchildren would also have to stick to online remote learning.

Nepal has resumed free tests and treatment after a court order and as new infections and deaths have increased across the Himalayan country. The government in Kathmandu had been providing free tests and treatment to all virus patients up until a month ago. Because of the deepening economic crisis, with tourism and remittances severely curtailed, the government instructed state-run hospitals and labs to begin collecting fees. A group of lawyers appealed to the Supreme Court, which last week ruled that the government must bear all expenses related to coronavirus tests and treatment.





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