Why the number of asymptomatic coronavirus cases matters

A significant number of infected people never feel sick at all. This is good news and bad news.

Maribel Bonilla was first in line at a pop-up food pantry at Bosson Park in Chelsea, on Wednesday.Craig F. Walker/Globe Staff

Coronavirus testing efforts in Boston and Chelsea led researchers to a startling discovery: A large number of people have been walking around with COVID-19 and don’t even know it. In fact, they may run the entire course of the virus without ever experiencing the slightest of symptoms — even as they travel in their communities and potentially infect relatives and strangers.

In Boston, health officials set out to see how the virus was spreading among the homeless population. What they did not expect to find was that over half the people who tested positive did not feel sick at all. And last week, researchers in Chelsea found a similar trend. Of 200 passersby on the street who agreed to have their blood samples taken, 63 tested positive for antibodies that suggested they had been sick with COVID-19. Of those, 25 said they hadn’t felt sick at all.

These so-called asymptomatic cases have wide-ranging implications for what we know about how the virus is spreading. They reveal the true, overwhelming scope. But the fact that so many people have already been infected — and felt fine — also provides a glimmer of hope.

What asymptomatic cases can tell us about the state of the pandemic

First, the bad news:

A high number of asymptomatic cases means the coronavirus has likely spread farther and faster than official data suggest, with many cases flying under the radar. It also means containing the virus will be difficult.

“This makes stamping out transmission basically impossible,” said Dr. Sarah Fortune, chair of the department of immunology and infectious diseases at Harvard T.H. Chan School of Public Health. People without symptoms are less likely to self-quarantine and more likely to spread the virus unwittingly.

Even if COVID-19 does not make an infected person sick, “it’s still dangerous for their family members who are at risk, or their neighbors or their co-workers,” said Dr. John Iafrate, vice chairman of Massachusetts General Hospital’s pathology department and the principal investigator behind a study that revealed asymptomatic cases in Chelsea.

But there is good news, too.

If coronavirus cases are more common than originally thought — and in many cases far less severe — communities with high rates of infection might already be past the worst of the pandemic. “It does suggest [Massachusetts] might be closer to the end,” Fortune said. “It’s fair to say we are probably at the top of a flattened peak.”

Moving toward herd immunity

Growing awareness of asymptomatic cases is closely tied to researchers’ and public health officials’ understanding of one important factor in curbing the pandemic: immunity.

People become immune to an infectious disease by either being vaccinated or surviving infection. But once enough individuals in a community are immune, the spread of the disease slows almost to a halt — even among those who have not been inoculated. This is what epidemiologists call herd immunity.

“Once you have enough people infected" in a community, Iafrate explained, "it’s difficult for the virus to get back in.”

Herd immunity is an elusive goal, but the prevalence of asymptomatic cases gives some reason to believe we can reach it — without everyone suffering critical illness, or any sickness at all. Even when infected people do not experience symptoms, they build antibodies that help fight the infection.

“That is good news for an individual and for the idea of herd immunity in their city," said Iafrate.

But many questions remain about immunity to COVID-19. “What we really need to understand is how strong is that immune response that can kill the virus, and how long does it last before there’s a risk for reinfection," said Dr. David Hamer, an infectious disease expert at Boston Medical Center and a professor at Boston University. Unfortunately, he continued, “We don’t really have a lot of information.”

Residents of Chelsea lineup to get a rapid fingerstick blood test from Partners Healthcare workers for the coronavirus.Stan Grossfeld/ Globe Staff

It also remains unclear exactly what it would take to reach herd immunity. “We don’t know what level of immunity we need to have in a population to protect against COVID," said Dr. Vivek Naranbhai, a clinical fellow in hematology and oncology who works alongside Iafrate.

What is considered herd immunity varies from disease to disease based on how easily infected people pass the disease along to others. For highly contagious measles, for example, 95 percent of people must be immune to protect the entire population. But until we know exactly how easily COVID-19 is transmitted, we cannot know what herd immunity requires.

Social distancing is still important

That many people have already been infected without experiencing symptoms does not mean it is safe to end preventive measures. "We still need to keep social distancing in place,” said Hamer.

Even if people with antibodies could be assured of their own immunity, they would still risk passing COVID-19 on to others. That is because people who test positive for antibodies can still test positive for the virus, meaning they remain infected and infectious. “Antibody tests don’t give you a ticket to change your protective behavior," Naranbhai said.

Even if new information about testing cannot bring an abrupt end to social distancing, it does provide public officials with invaluable information about the true scope of the virus.

“This information has helped the city of Chelsea mobilize more resources,” said Naranbhai. Since learning that infections were even more widespread than previously thought, he said, city officials have worked to provide services like food delivery and safe housing to residents for whom social distancing had been difficult.

Those measures pave the way for ending the pandemic. “If we can take even just a little bit off the top of the curve," Naranbhai said, “that’s what this is all about.”