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 even in results from a new set of studies that test the population more broadly to estimate everyone who has been infected, 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 percent.
Herd immunity estimate
At least 60% of population
New York CityMay 2
19.9% have antibodies
17.5% have antibodies
11.3% have antibodies
Wuhan (returning workers)April 20
10% have antibodies
9.9% have antibodies
7.3% have antibodies
7.1% have antibodies
Even in some of the hardest-hit cities in the world, the studies suggest, the 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 7 to 17 percent of people have been infected so far. In New York City, which has had the largest coronavirus outbreak in the United States, around 20 percent 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 centers 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 percent 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 and 80 percent of the population have been infected and develop 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,” 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.”
The new studies look for antibodies in people’s blood, proteins produced by the immune system that indicate a past infection. An advantage of this test is that it can capture people who may have been asymptomatic and didn’t know they were sick. A disadvantage is that the tests are sometimes wrong — and several studies, including a notable one in California, have been criticized for not accounting for the possibility of inaccurate results or not representing the whole population.
Studies that use these tests to examine a cross section of a population, often called serology surveys, are being undertaken around the country and the world.
While these studies are far from perfect, said Carl Bergstrom, a professor of biology at the University of Washington, in aggregate they give a better sense of how far the coronavirus has truly spread – and its potential for spreading further.
The herd immunity threshold may differ from place to place, depending on factors like density and social interaction, he said. But, on average, experts say it will require at least 60 percent immunity in the population. If the disease spreads more easily than is currently believed, the number could be higher. If there is a lot of variation in different people’s likelihood of becoming infected when they are exposed, that could push the number down.
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.
Dr. Mina of Harvard said to think about immunity in the population 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.
We don’t have a 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.
If you assume that herd protection could be achieved when 60 percent of the population becomes resistant to the virus, that means New York City is only one-third of the way there. And, so far, nearly 250 out of every 100,000 city residents has died. New York City still has millions of residents vulnerable to catching and spreading this disease, and tens of thousands more who are at risk of dying.
“Would someone advise that people go through something like what New York went through?” said Natalie Dean, an assistant professor of biostatistics at the University of Florida. “There’s a lot of people who talk about this managed infection of young people, but it just feels like hubris to think you can manage this virus. It’s very hard to manage.”
In other cities, serology surveys are showing even smaller shares of people with antibodies. The quality of these studies is somewhat varied, either because the samples weren’t random or because the tests were not accurate enough. But the range of studies shows that most places would have to see 10 or more times as many illnesses – and possibly, deaths – to reach the point where an outbreak would not be able to take off.
The serology studies can also help scientists determine how deadly the virus really is. Currently, estimates for what’s called the infection fatality rate are rough. To calculate them precisely, it’s important to know how many people in a place died from the virus versus how many were infected. Official case rates, which rely on testing, undercount the true extent of infections in the population. Serology helps us see the true footprint of the outbreak.
In New York City, where 20 percent of people were infected with the virus by May 2, according to antibody testing, and more than 18,000 had died by then, the infection fatality rate appears to be around 1 percent.
For comparison, the infection fatality rate for influenza is estimated at 0.1 percent to 0.2 percent. But the way the government estimates flu cases every year is less precise than using serology tests and tends to undercount the number of infections, skewing the fatality number higher.
But even if the fatality rates were identical, Covid-19 would be a much more dangerous disease than influenza. It has to do with the number of people who are at risk of getting sick and dying as the disease spreads.
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. But there are benefits to partial immunity in the population: Only a fraction of adults are at risk of catching the flu in a normal year, and they can spread it less quickly, too. That means that the number of people at risk of dying is also much lower.
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. One percent of a large number is bigger than 1 percent 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.”