Asymptomatic transmission is possible, but the World Health Organization is still unsure as to what extent asymptomatic COVID-19 infections contribute to the spread of the virus, according to WHO Director-General Dr Tedros Adhanom Ghebreyesus on Wednesday.

“Since early February, we have said that there are asymptomatic people that can transmit COVID-19. But we need more research to establish the extent of asymptomatic transmission, and that research is ongoing,” said Dr Tedros.

The WHO Director-General attempted to clarify seemingly contradicting statements made by WHO COVID-19 Technical Lead Maria Van Kerkhove on Tuesday and Monday regarding asymptomatic transmission, the onwards spread of COVID-19 by an infected person who never develops symptoms.

Van Kerkhove had on Monday said that transmission of COVID-19 by asymptomatic people was “very rare,” according to reports from countries doing detailed contact tracing 

“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… It is very rare,” said Van Kherkhove at WHO’s regular press briefing on Monday. “From the data we have it still seems to be rare that an asymptomatic person actually transmits onward to a secondary individual.”

The comment seemed to contradict United States Centers for Disease Control guidelines that advise pandemic preparedness teams to assume that 35% of all COVID-19 infections are asymptomatic, and asymptomatic persons are just as likely to transmit the infection as symptomatic persons. The modeling guidelines are based on COVID-19 estimates from the US CDC, and US Health and Human Services Office of the Assistant Secretary for Preparedness and Response (ASPR).

WHO COVID-19 Technical Lead Clarifies “Rare” Asymptomatic Transmission Comment After Concerns 

Van Kerkhove walked back on her comments on Tuesday, after receiving backlash from scientists concerned that WHO was writing off the threat of asymptomatic transmission.

She clarified that her comment designating asymptomatic transmission as “rare” was based on a small subset of 2 to 3 studies following asymptomatic people and their contacts, and the true extent of asymptomatic transmission was still unknown.

“”The majority of transmission that we know about is that people who have symptoms transmit the virus to other people through infectious droplets. There are a subset of people who do not develop symptoms,” said Van Kerkhove at a Twitter livestream on the issue on Tuesday. “To truly understand how many people don’t develop symptoms – we don’t have that answer yet.

“I wasn’t stating a policy of WHO or anything like that, I was just trying to articulate what we know. In that, I used the phrase ‘very rare’ – and I think that’s misunderstanding to state that asymptomatic transmission globally is very rare.”

Van Kerkhove highlighted that due to the many unknowns, infectious disease modeling groups have used their own estimates of the contribution of asymptomatic transmission.

“What I didn’t report yesterday, was because this is a major unknown, some modeling groups have tried to estimate what is the proportion of asymptomatic people that may transmit,” she said. “And there is a big range, depending on how the models are done, from which country…They can be [quite high].

“Some estimates are around 40% of transmission may be due to asymptomatic [infections],” said Van Kerkhove, a potential nod to the CDC modeling guidelines.