Here’s What We Know About Asymptomatic COVID-19 Carriers

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In early April, the Centers for Disease Control and Prevention issued new face-mask guidance, encouraging everyone to wear cloth face coverings when in public. The reasoning: “We now know from recent studies that a significant portion of individuals with coronavirus lack symptoms” and that those individuals can transmit the virus to others.

But in a statement in June, the World Health Organization said that it believes asymptomatic transmission to be “very rare,” particularly among young, otherwise healthy people — a seemingly dramatic reversal from a few months ago, when social distancing was enacted largely as a precaution against asymptomatic transmission.

The WHO’s comments led to a flurry of news items — and some pushback from concerned health experts. In an attempt to clear up confusion, the WHO held another press conference the next day, at which Dr. Maria Van Kerkhove, head of the emerging diseases and zoonoses unit, clarified that her statement was based on a small number of contact-tracing studies done in China. While experts still believe that most transmission occurs through people with coronavirus symptoms, she said, the WHO estimates that 16 percent of asymptomatic infected people can transmit COVID-19, and unpublished disease models suggest that number could be as high as 40 percent.

While we don’t know exactly how many people infected with the coronavirus are truly asymptomatic, the CDC has estimated that it’s around 35 percent, and in June, the journal Nature Medicine published a report saying those who are asymptomatic might still sustain lung damage. The availability of testing continues to improve, though we still don’t have a clear picture as to exactly how many American cases are asymptomatic, or what an asymptomatic case could mean for one’s future health. But here’s what we know about asymptomatic carriers so far.

Just to be clear: What exactly does it mean to be asymptomatic?

If you are truly asymptomatic, that means you are infected with a virus but you never exhibit symptoms of disease. In the case of the coronavirus, those common symptoms would be dry cough, fever, and fatigue but might also include aches, gastrointestinal distress, and congestion. However, as health officials have repeatedly stressed in an attempt to convince people to adhere to strict social-distancing measures, people with the coronavirus who are asymptomatic can be contagious.

How many people with the coronavirus are asymptomatic?

Estimates vary pretty widely. Some studies are reporting alarmingly high numbers: In early April, one lab in Iceland reported that as many as 50 percent of cases could be asymptomatic; in the same month, India’s top medical research body reported that out of 100 infected people they studied, 80 did not have symptoms. In one Boston homeless shelter, where 400 guests were staying, 146 tested positive for COVID-19 in April, all of whom were reported to be asymptomatic.

So what are health officials saying? In April, CDC director Dr. Robert Redfield estimated that 25 percent of people with the coronavirus may be asymptomatic, but the agency has since increased that number to 35 percent. Additionally, National Institute of Allergy and Infectious Diseases director Dr. Anthony Fauci has estimated that 25 to 50 percent of cases may be asymptomatic.

Is it possible that some people who were asymptomatic when they were tested later showed symptoms?

Absolutely. Van Kerkhove has told ProPublica she thinks that many cases have been misclassified as asymptomatic when in fact they were presymptomatic. (People who are presymptomatic have no symptoms when they test positive but go on to develop symptoms.) Part of the persisting confusion here stems from the fact that some people who do have mild symptoms might not fully register that they have symptoms, Van Kerkhove said at the Tuesday press conference.

Some examples of this have been documented: The CDC, for example, found that of the 13 patients in a nursing facility in Washington State who reported no symptoms when they tested positive for the coronavirus, ten went on to later develop symptoms. It’s also possible that some people who have been tested are underplaying their own symptoms or simply don’t register them. “Most of the people who were thought to be asymptomatic aren’t truly asymptomatic,” said Van Kerkhove. “When [WHO] went back and interviewed them, most of them said, ‘Actually, I didn’t feel well, but I didn’t think it was an important thing to mention. I had a low-grade temperature, or aches, but I didn’t think that counted.’”

Dr. Jeffrey Shaman, an infectious-diseases expert at Columbia University, has cautioned against spending too much time on the semantics debate between asymptomatic and presymptomatic. He told the New York Times, “The bottom line is that there are people out there shedding the virus who don’t know that they’re infected.”

Can asymptomatic people still sustain lung damage?

From what we can tell, it is possible. According to a study published in the journal Nature Medicine in June, as reported by NPR, the virus can still cause mild damage even in those who are asymptomatic. The study looked at the clinical and immunological patterns of those with asymptomatic cases, and found that many developed inflamed lungs even as they showed no other symptoms. As NPR notes, though, asymptomatic carriers of things like the flu or common cold are not often studied. Because of that, we don’t know for sure whether the lung inflammation is a typical immune response or something specific to this virus.

This post has been updated.

Here’s What We Know About Asymptomatic COVID-19 Carriers