NEW YORK (NYTIMES) - The coronavirus still has a long way to go. That's 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, at which the virus can no longer spread widely. The precise herd immunity threshold for the novel 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 still remain vulnerable to the virus.
Some countries - notably Sweden, and briefly Britain - have experimented with limited lockdowns in an effort to build up immunity in their populations. But even in these places, recent studies indicate that no more than about 7 per cent -17 per cent of people have been infected so far. In New York City, which has had the largest coronavirus outbreak in the United States, around 20 per cent of the city's 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 underway 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 with the virus.
Viewed together, the studies show herd immunity protection is unlikely to be reached "any time soon," said Michael Mina, an epidemiologist at the Harvard T.H. Chan School of Public Health.
The herd immunity threshold for this new disease is still uncertain, but many epidemiologists believe it 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 slow the spread of a disease somewhat, 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," 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 whether that is true in all cases; how robust the immunity may be; or how long it will last.
Mina of Harvard suggested thinking about population immunity as a firebreak, slowing the spread of the disease.
If you are infected with the virus 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. And that makes it much less likely that a large outbreak can bloom.
Even with herd immunity, some people will still get sick. "Your own risk, if exposed, is the same," said 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 United States because vaccines have helped build enough herd immunity to contain outbreaks. There is no vaccine for the coronavirus, so getting to herd immunity without a new and more effective treatment could mean many more infections and many more deaths.
With the flu, only about half the population is at risk of getting sick in a given flu season. Many people have some immunity already, either because they have been sick with a similar strain of flu, or because they got a flu shot that was a good match for the version of the virus they encountered that year.
That number isn't high enough to fully reach herd immunity - and the flu still circulates every year.
Covid-19, unlike influenza, is a brand-new disease. Before this year, 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, as 1 per cent percent of a large number is bigger than 1 per cent of a smaller number.
"There aren't 328 million Americans who are susceptible to the flu every fall at the beginning of the flu season," said Andrew Noymer, an associate professor of public health at the University of California, Irvine. "But there are 328 million Americans who were susceptible to this when this started."
