WHO said on Monday that it's "rare" for an asymptomatic person who shows no tell-tale signs of coronavirus infection — like a fever, dry cough, or trouble breathing — to spread the disease to others.
Other public-health experts were quick to point out that studies so far have shown pre-symptomatic people and asymptomatic people can spread the virus.
That's the reason people are being urged to wear masks in public.
WHO did not dismiss the idea of asymptomatic spread entirely but said "from the data we have, it still seems to be rare that an asymptomatic person actually transmits onward to a secondary individual."
The World Health Organization said Monday that the threat of coronavirus spread from people who look and feel healthy is very low.
"It still seems to be rare that an asymptomatic person actually transmits onward," epidemiologist Maria Van Kerkhove, WHO's technical lead for the coronavirus, told reporters on a call.
The comment falls in line with what experts at WHO have been stressing for months: Most coronavirus spread happens through sustained close contact between people indoors — in homes, offices, churches, hospitals, and other settings where people come together for hours at a time and airflow is restricted.
The notion that perhaps healthy, or seemingly healthy, people are not a threat during the pandemic has huge implications. If asymptomatic carriers are not a big threat, then face coverings in public become much less important because only sick people displaying symptoms should need to wear them.
In its recent updated guidance on mask wearing, WHO said studies of asymptomatic transmission to date are small but that virus spread via fomites (objects and surfaces) in those cases cannot be ruled out.
Furthermore, Van Kerkhove said, many cases that are labeled asymptomatic may not truly be that, but instead are mild — but detectable — illnesses.
This is why it may still be important for seemingly healthy people to wear face coverings in public. Some people may feel a little bit ill or tired but not enough to know they have the virus.
"When we actually go back and say, 'How many of them were truly asymptomatic?' we find out that many have really mild disease, very mild disease," Van Kerkhove said.
While WHO recognizes that some people infected with COVID-19 will never develop symptoms, and evidence suggests they can shed virus too, Van Kerkhove said this is not how most people are getting sick.
Other public-health experts say it's possible people may spread illness to others before they show symptoms
JOSH EDELSON/AFP via Getty Images
Other public-health experts, however, aren't as dismissive of asymptomatic spread, especially because of the possibility of pre-symptomatic spread that can happen early on in an illness, before people feel unwell.
"There's significant transmission by people not showing symptoms," Stephen Morse, an epidemiologist at Columbia University, previously told Business Insider. Even some of WHO's own research suggests many coronavirus patients who are initially asymptomatic eventually fall ill.
"Some modeling studies suggest 40-60% of spread is from people when they didn't have symptoms," Ashish Jha, the director of the Harvard Global Health Institute, said on Twitter. "It may be there isn't a lot of asymptomatic spread but plenty of pre-symptomatic spread. Would be helpful to get the full report they are referencing."
WHO said that kind of robust data just isn't available yet.
"We are constantly looking at this data, and we're trying to get more information from countries to truly answer this question," Van Kerkhove said on the call. "We have a number of reports from countries who are doing very detailed contact tracing. They're following asymptomatic cases, they're following contacts, and they're not finding secondary transmission onward. It's very rare. And much of that is not published in the literature."
Either way, Van Kerkhove seemed confident that by strictly isolating the people who are feeling sick, the world would do much better at stopping the spread of COVID-19.
"I would love to be able to give a proportion of how much transmission we would actually stop, but it would be a drastic reduction in transmission," she said. "From the data we have, it still seems to be rare that an asymptomatic person actually transmits onward to a secondary individual."
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