Five months after public health officials began to worry over the mysterious respiratory illness in China now known as Covid-19, the official death toll is close to 400,000 and millions have been infected.

An unprecedented global research effort has produced tens of thousands of scientific papers and is clarifying many aspects of the new coronavirus and the way it affects the human body. Much, however, remains obscure. Here we sum up what we know and what we do not.

How many people have already been infected?

As accurate antibody tests become available to detect signs of past infection, surveys around the world are beginning to provide more realistic figures than those incorporated in epidemiological models early in the pandemic.

Weekly testing by the UK Office for National Statistics (ONS) indicates that 6.8 per cent of the population in England has been infected (and thus developed antibodies to the virus). The latest national survey in Spain published last week indicated that 5.2 per cent of the population had been infected.

22.7%

of people in New York City have had Covid-19

The picture is different in large cities where Covid-19 spread fastest before lockdown, accelerated by high-density living. Matt Hancock, the UK health secretary, said 17 per cent of Londoners carried coronavirus antibodies. One of the highest confirmed rates was in New York City, where it hit 22.7 per cent, according to a study by NY State Department of Health and NY State University at Albany.

Such levels are nonetheless far below the 60 per cent needed for “herd immunity” to stop the virus from spreading. And anyway, as scientists emphasise, no one knows yet the strength of immune protection being detected or how long it will last.

What proportion of those infected come down with disease?

The Sars-Cov-2 virus responsible for Covid-19 infects most people without ever revealing its presence. Several studies show that roughly two-thirds of people who carry enough virus to test positive never develop symptoms during their infection. How infectious they are to others is not so clear.

70%

of people infected in UK show no symptoms

A study of the Diamond Princess cruise ship, which spent most of February in quarantine off Japan while the virus spread through passengers and crew, found that 72 per cent of infections were asymptomatic. The UK ONS survey yielded a similar result, finding that 70 per cent of people infected never showed any symptoms. Government data in China have indicated that 60 per cent of all coronavirus cases logged in April showed no symptoms.

How long does it take Covid-19 to harm the body?

The incubation period for patients who do develop symptoms is typically five to six days. But what seemed at first like a classic respiratory illness causing pneumonia turns out to be far more complex.

The severe form of Covid-19, which typically appears about a week after the first symptoms, now appears to be more of a vascular and inflammatory disease, though few tissues and organs are immune from its effects. Death can arrive as soon as two weeks after symptoms start or as long as eight weeks later. 

According to the World Health Organization, around 20 per cent of people with symptoms require hospital treatment. Milder effects include losing the capacity to smell and taste, sore and swollen skin, fever and lethargy. But even these can last for several weeks, though the active virus may no longer be present.

What is the fatality rate?

It has been hard to give a realistic estimate for the proportion of people with Covid-19 who die from the disease. If you take the overall numbers of confirmed cases (6.2m) and deaths (370,000) worldwide, you come to a scarily high “case fatality rate” near 6 per cent.

Officially recorded cases exclude the far larger numbers who were infected but not tested because their symptoms were non-existent or mild. That total is unknown but runs to tens of millions of people around the world, according to mathematical modelling based on the available statistics.

0.5 – 1%

of those infected have died from Covid-19

The “infection fatality rate” — the proportion of those infected who die — depends on local circumstances but is typically in the 0.5 to 1 per cent range. A study by Imperial College London of the epidemic in China found an IFR of 0.66 per cent. Meta-analysis by Australian epidemiologists who pulled together 25 studies around the world calculated that the average IFR was 0.64 per cent.

But an individual’s risk of dying depends on a vast range of factors, ranging from age and sex to underlying health conditions such as diabetes and heart disease, as well as the circumstances of infection and quality of medical care.

In Italy, just 365 people, or 1.1 per cent, of the 32,448 confirmed to have died with the virus as of June 4 were under the age of 50, according to the ministry of health. A study of 20,000 UK patients admitted to hospital with Covid-19 found that the median age of those who died of the disease was 80 years; some 89 per cent of those patients were already suffering from serious chronic conditions. 

How fast is the virus spreading?

The now-famous reproduction number R measures the average number of new cases generated by each infected individual.

R 0.7-0.9

Likely transmission rate of coronavirus in UK

Although R0, the basic reproduction number in a population with no immunity and no containment measures, may have been above 3 in the early stages of the epidemic in Europe and North America, social distancing has pushed R below 1 in most countries. The UK government’s science advisers estimate that it is currently between 0.7 and 0.9.

When the transmission is very low, an epidemic can decline fast. The best example is New Zealand, which eliminated the virus within a few weeks by maintaining R below 0.5.

How about superspreaders?

Epidemiologists are increasingly discussing another number, the “dispersion parameter” k, as a key factor in Covid-19 transmission. This measures how uniformly an infection spreads: a higher k means less variation between the way individuals transmit the virus. 

Superspreaders cause a disproportionate number of cases73

People infected in Hong Kong by one superspreader

10-20%

of those infected account for the vast majority of transmissions

50

People infected in Seoul’s bars by one person

In a flu pandemic k is estimated at around 1 but for coronavirus it is believed to be much lower, meaning that transmission is very uneven. Most people do not pass Sars-Cov-2 virus on to anyone else, while a few superspreaders may transmit it to dozens of others. Emerging evidence suggests that 10 to 20 cent of Covid-19 cases account for 80 per cent of transmission.

Countries with good track and trace systems have detected some remarkable superspreaders, including one person who infected 73 others in several Hong Kong bars in March and another who infected more than 50 people in Seoul bars last month.

Additional reporting by Davide Ghiglione in Rome and Daniel Dombey in Madrid

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