What You Need to Know About Asymptomatic Spread of Covid-19

Nearly six months have passed since Chinese officials first reported the emergence of a strange new pneumonia in Wuhan City. Despite months of concerted effort from the world’s scientific community, experts still aren’t certain just how the virus spreads — or who is capable of spreading it.

During a media briefing on June 8, an official at the World Health Organization set off a broad volley of expert rebukes when she said that it “appears to be rare” for an asymptomatic person to transmit the virus to others.

“We have a number of reports from countries who are doing very detailed contact tracing,” said the WHO official, Maria Van Kerkhove, PhD, who is that organization’s Covid-19 technical lead. “They’re following asymptomatic cases, they’re following contacts, and they’re not finding secondary transmission onward. It is very rare.”

In addition to Van Kerkhove’s comments, the WHO also published a report that stated: “Asymptomatically-infected individuals are much less likely to transmit the virus than those who develop symptoms.”

“The message should have been that symptom-free spread has been difficult to detect via contact tracing, not that it isn’t happening.”

If people who do not develop noticeable symptoms are unlikely to spread Covid-19 to others, that would be a significant revelation. It would radically change the way governments and health officials approach quarantines and closures, and it would make tracing and controlling the virus much simpler. Unfortunately, most experts feel there is little evidence to support the WHO’s statements — which the organization itself partly retracted the day after its controversial briefing.

“It was all really unfortunate because it engendered a lot of confusion,” says Eric Topol, MD, a professor of molecular medicine at the Scripps Research Institute in California. “We know that asymptomatic people can infect others.”

Topol has published work on those who contract Covid-19 but do not develop symptoms, and on the “silent spread” that they may unwittingly induce. He says that it’s not clear what proportion of cases stem from people who do not have symptoms, a group that may include 40% or more of those infected with Covid-19. The WHO report mentions one study of 63 asymptomatic patients in China that found that 14% seemed to have passed on Covid-19 to others. But the WHO says that the data on this is poor due in part to the limitations of contact-tracing techniques, which attempt to track a virus by monitoring those who have been in contact with someone who tests positive.

This is an important qualification that other experts echo. “The message should have been that symptom-free spread has been difficult to detect via contact tracing, not that it isn’t happening,” says Jeremy Faust, MD, an instructor at Harvard Medical School and an emergency medicine physician at Brigham and Women’s Hospital.

Faust has published research comparing Covid-19 to seasonal flu. He explains that the WHO’s contract tracing procedures call for monitoring people who are in touch with a Covid-19 carrier within 48 hours of either symptom emergence or a positive test result. “That’s a great approach in infections that do not spread asymptomatically or for very long. SARS-CoV-2, unfortunately, does not play by those rules,” he explains. “Some people shed virus and are contagious further back than just two days before first symptoms.” It’s also very possible that those who test positive for Covid-19 are spreading the virus outside of the WHO’s 48-hour window.

Scripps’s Topol says that another problem surrounding asymptomatic cases is that a lot of the messaging from doctors and the media has unhelpfully downplayed the potential seriousness of these infections. Just because someone is asymptomatic does not mean that the virus is not doing damage.“If you do a CAT scan of these patients’ lungs, you see significant abnormalities tied to Covid, and these are woefully understudied,” he says. “We also know this the virus can go to the heart and kidneys, so that needs to be looked at too.” It’s possible, he adds, that these “below-the-surface problems” may increase a person’s risk for long-term health complications, such as the eventual development of lung disease.

Finally, he points out an issue that has more to do with media coverage of the WHO statements than with the statements themselves. Van Kerkhove, before saying that asymptomatic transmission may be “very rare,” took pains to point out that there’s a difference between people who are asymptomatic — meaning those who never develop noticeable symptoms — and those who are presymptomatic, which refers to those who are infected with Covid-19 and will eventually go on to show symptoms. According to the WHO, it may take as long as 14 days for these presymptomatic carriers to become symptomatic.

“We know presymptomatic people are highly infectious, and they’re responsible for a lot of the virus’s spread,” Topol says. In fact, there’s some evidence that these presymptomatic people may be even more likely to transmit the disease than people who have symptoms, he says. And so the idea that people who don’t have symptoms are unlikely to spread the virus is flawed and dangerous on many levels.

“We’re still in the dark on a lot of things,” Topol adds. “There are a lot of very important questions we need to answer.”