Medical staff prepared to deliver either a placebo or the Moderna vaccine candidate at a hospital in Miami in September.Credit…Taimy Alvarez/Associated Press

The scientists randomly assigned volunteers to get either the Moderna vaccine or a placebo. The trial was blinded, meaning that neither the volunteers nor the people running the trial knew who got what.

Over time, some of the volunteers got sick with Covid-19. The company reported on Nov. 30 that of 196 cases, 185 occurred in the placebo group and only 11 in those who were vaccinated. Based on that data, the vaccine was found to be 94.1 percent effective. That figure is in keeping with an earlier analysis based on 95 cases, which found the vaccine 94.5 percent effective.

A concertgoer in Red Bank, N.J., this summer. Vaccine development is promising, but it will still be months before immunizations are widely available.Credit…Mark Makela for The New York Times

In the short term, no. The soonest that coronavirus vaccines could possibly become widely available would be in the spring. But if effective vaccines do indeed become available — and if most people get them — the pandemic could drastically shrink. If coronavirus infections became rarer, life could gradually return to normal.

An injection during a safety study of Moderna’s vaccine candidate.Credit…Ted S. Warren/Associated Press

Covid-19 can lead to a mild illness, or it can lead to a severe case that requires hospitalization and oxygen support. Of the 196 people who got sick in the Moderna study, 30 experienced severe disease. One died. None of those 30 people were vaccinated. In other words, the 11 vaccinated people who got sick experienced only mild symptoms, and all of the severe cases were participants from the placebo group.

“It couldn’t be a more favorable split,” said Natalie Dean, a biostatistician at the University of Florida.

The split suggests that Moderna’s vaccine doesn’t just block the virus in most cases, but it also shields the people who do get sick from the worst outcomes of the disease. It also eases concerns that a vaccine for Covid-19 may make the disease worse, not better.

A participant in Moderna’s vaccine trial received a nose swab at a hospital in Miami.Credit…Taimy Alvarez/Associated Press

Moderna recruited 30,000 volunteers across the United States to participate in its trial. A quarter of the participants are 65 years or older. White people make up 63 percent of the volunteers; 20 percent are Hispanic; 10 percent are Black; and 4 percent are Asian-Americans.

The 196 people who got sick with Covid-19 reflect the diversity of Moderna’s volunteers: 33 were 65 or older. The group also included 29 Hispanic volunteers, six Black participants, four Asian-Americans and three multiracial people. The efficacy and safety appeared the same in all of the subgroups, Moderna said in its announcement.

Dr. Moncef Slaoui, the head of Operation Warp Speed, gave an update from the Rose Garden of the White House on Nov. 13.Credit…Anna Moneymaker for The New York Times

[Read more on Moderna’s Covid-19 vaccine.]

Very much so. The United States government provided $1 billion in support for the design and testing of the Moderna vaccine. Researchers at the National Institutes of Health oversaw much of the research, including the clinical trials. Moderna also received an additional $1.5 billion in exchange for 100 million doses if the vaccine proved to be safe and effective.

Although Pfizer has its own advance purchase agreement for its vaccine, it did not take money from Operation Warp Speed to support its design or testing.

Outside Pfizer’s headquarters in Manhattan.Credit…Carlo Allegri/Reuters

[Read more on Moderna’s Covid-19 vaccine.]

Pfizer has reported that its vaccine is 95 percent effective, so the estimates for the two vaccines are clearly in the same ballpark.

What’s more, they both far exceed the F.D.A.’s requirement that coronavirus vaccines have an efficacy of more than 50 percent. Pfizer also reported that its vaccine prevented severe disease: Of 10 severe cases in its large study, nine were in the placebo group.

Research on Russia’s Sputnik V vaccine earlier this year.Credit…Russian Direct Investment Fund/EPA, via Shutterstock

The Pfizer and Moderna vaccines are similar not only because they use mRNA, but also because they coax our cells to make the same viral protein, called spike. Other vaccines that don’t use mRNA also make the spike protein their target. The success of Moderna and Pfizer may bode well for them as well.

A number of teams have created vaccines based on another virus called an adenovirus, for example. The adenovirus slips into cells, delivering the gene for the spike protein. Earlier in November, a sponsor of a Russian vaccine announced that its adenovirus-based vaccine, called Sputnik V, was over 90 percent effective. Outside experts wanted to see more data, however, because the announcement was based on just 20 sick volunteers — far fewer than in the Moderna and Pfizer trials.

AstraZeneca and Johnson & Johnson are also conducting Phase 3 trials on adenoviruses that carry the spike protein gene. AstraZeneca reported promising but puzzling results: Its vaccine was 62 percent effective in people who received two doses, but 90 percent effective in those whose first shot mistakenly contained only half a dose. More data are needed to interpret the findings.

Other companies, including Novavax and Medicago, are running advanced trials on vaccines that deliver the spike protein itself, or pieces of it, to the body.

Empty collection vials for taking patients’ blood in the Moderna vaccine trial.Credit…Taimy Alvarez/Associated Press

We don’t know. Both Moderna and Pfizer started their trials on July 27, so they have been able to follow their volunteers for only a few months so far.

It’s conceivable that the vaccines provide long-lasting protection, or fade away in under a year and require a booster.

Trial kits for Pfizer’s vaccine study at the Research Centers of America in Hollywood, Fla.Credit…Marco Bello/Reuters

Pfizer and Moderna used the same basic design to build their vaccines. Both vaccines contain a genetic molecule called messenger RNA, which is wrapped in an oily bubble. The bubble can fuse to a muscle cell and deliver the RNA. Encoded in that molecule are instructions for building a single coronavirus protein called spike. When a vaccinated cell releases copies of the spike protein, the immune system learns to make antibodies against it.

While scientists have investigated mRNA vaccines for years, no vaccine has yet been licensed as safe and effective to use in people. When Moderna and other vaccine makers began designing mRNA vaccines for coronaviruses, skeptics wondered how well they would work. The two preliminary reports from both Moderna and Pfizer suggest this type of vaccine may work very well. Neither trial has uncovered serious side effects from the vaccines, although studies on their safety are continuing.

“I would expect some similarities in how they perform,” said Natalie Dean, a biostatistician at the University of Florida. “I think I would have had a lot of questions if they got different results.”

Construction on a building in Visp, Switzerland, where the Moderna vaccine will be produced.Credit…Denis Balibouse/Reuters

Both Moderna and Pfizer have applied to the Food and Drug Administration for an emergency use authorization to begin vaccinating the public.

The F.D.A. will review the applications and consult with its own external committee of experts before making a decision. If it authorize the vaccines — as experts think it will — a committee at the Centers for Disease Control and Prevention will then make recommendations for who should be first to receive a vaccine.

The first vaccinations could begin before the end of the year.

Even if approved soon, vaccines will not be widely available to make traveling for the holidays safe.Credit…Chang W. Lee/The New York Times

The vaccines will not reduce the risk of infections over the holiday season.

Even if the authorization of Moderna and Pfizer’s vaccines comes quickly, only a tiny fraction of Americans will be vaccinated by the end of December. Vaccines won’t enter large-scale distribution until spring 2021.

But with exploding cases across the country right now, we have to take immediate measures. The only way to drive down infection rates for now will be to avoid large indoor gatherings, wear masks, practice physical distancing and use other public health measures.

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