NEW YORK • The coronavirus still has a long way to go. That is the message from a crop of new studies across the world that are trying to quantify how many people have been infected.
Official case counts often substantially underestimate the number of coronavirus infections. But in new studies that test the population more broadly, the percentage of people who have been infected so far is still in the single digits.
The numbers are a fraction of the threshold known as herd immunity, the point at which the virus can no longer spread widely. The precise herd immunity threshold for the new coronavirus is not yet clear; but several experts said they believed it would be higher than 60 per cent.
Even in some of the hardest-hit cities in the world, the studies suggest, a vast majority of people remain vulnerable to the virus.
Some countries - notably Sweden, and briefly Britain - have experimented with limited lockdowns to build up immunity in their populations. But even in these places, recent studies indicate that no more than about 7 per cent to 17 per cent of people have been infected so far.
In New York City, which has had the largest Covid-19 outbreak in the United States, around 20 per cent of its residents have been infected by the virus as of early May, according to a survey of people in grocery stores and community centres released by the governor's office.
Similar surveys are under way in China, where the coronavirus first emerged, but results have not yet been reported. A study from a single hospital in the city of Wuhan found that about 10 per cent of people seeking to go back to work had been infected.
Viewed together, the studies show herd immunity protection is unlikely to be reached "any time soon", said Dr Michael Mina, an epidemiologist at Harvard T.H. Chan School of Public Health.
Many epidemiologists believe herd immunity for the coronavirus will be reached when between 60 per cent and 80 per cent of the population has been infected and develops resistance.
A lower level of immunity in the population can help slow the spread of a disease, but the herd immunity number represents the point where infections are substantially less likely to turn into large outbreaks.
"We don't have a good way to safely build it up, to be honest, not in the short term," Dr Mina said. "Unless we're going to let the virus run rampant again - but I think society has decided that is not an approach available to us."
All estimates of herd immunity assume that a past infection will protect people from becoming sick a second time. There is suggestive evidence that people do achieve immunity to the coronavirus, but it is not yet certain if that is true in all cases, how robust the immunity may be, or how long it will last.
Dr Mina suggested thinking about population immunity as a firebreak, slowing the spread of the disease. If you are infected and walk into a room where everyone is susceptible to it, he said, you might infect two or three other people on average.
"On the other hand, if you go in and three out of four people are already immune, then on average you will infect one person or fewer in that room," he said. That person in turn would be able to infect fewer new people, too.
"Your own risk, if exposed, is the same," said Dr Gypsyamber D'Souza, a professor of epidemiology at Johns Hopkins University. "You just become much less likely to be exposed."
Diseases like measles and chickenpox, once very common among children, are now extremely rare in the US because vaccines have helped build enough herd immunity to contain outbreaks. There is no vaccine for the coronavirus yet, so getting to herd immunity without a new and more effective treatment could mean many more deaths.
With the flu, only about half the population is at risk of getting sick in a given influenza season. Many people have some immunity already, either because they have been sick with a similar strain, or because they had a flu shot.
Covid-19, unlike influenza, is a brand-new disease. Before the current outbreak, no one in the world had any immunity to it at all. And that means that, even if infection fatality rates were similar, it has the potential to kill many more people.
NYTIMES
