For months, researchers have warned that people without any COVID-19 symptoms could still be silent carriers of the disease, making it that much harder to get the pandemic under control—and that much more important to take precautions like social distancing and wearing a mask, even if you feel fine.
So it came as a surprise when Maria Van Kerkhove, the World Health Organization’s (WHO) technical lead for COVID-19, said at a press briefing on June 8 that asymptomatic transmission appears to be “very rare.” Her statement came just days after the organization directed healthy people living in areas with widespread community transmission to wear fabric face masks in public to help contain the advance of the disease.
In an interview with TIME following the press briefing, Van Kerkhove said she did not mean to suggest that asymptomatic people cannot spread COVID-19. “I did not say that asymptomatic cases cannot transmit; they can,” Van Kerkhove says. “The question is, do they? And if they do, how often is that happening?”
Van Kerkhove says there’s not yet a clear answer, but the WHO’s analyses suggest symptomatic individuals are responsible for most coronavirus transmission. (She also clarified during a June 9 briefing that her comments were in response to a journalist’s question, and did not constitute official WHO policy.) The WHO laid out its thinking in its latest guidance on face masks, which was circulated on June 5, and was based on a number of reports that examined COVID-19 community spread and transmission dynamics, as well as not-yet-published findings from contact-tracing reports from multiple WHO member states.
Few of the cited papers explicitly examined population-level asymptomatic transmission rates. One, a preprint (i.e., not-yet peer-reviewed) research review posted to the site MedRxiv on June 4, analyzed four previous studies (two published and two preprint) that estimated asymptomatic transmission rates. The highest estimate was a transmission rate of 2.2%, suggesting “asymptomatic spread is unlikely to be a major driver of clusters or community transmission of infection.”
The WHO’s guidance also notes that some studies that have found evidence for asymptomatic transmission had small sample sizes, which would make their findings less statistically relevant. In addition, the WHO said, some of these studies did not rule out alternative explanations for how some patients may have contracted the virus, like touching a contaminated surface.
However, just last week, researchers from the Scripps Research Translational Institute published a paper estimating that asymptomatic individuals account for up to 45% of coronavirus cases, and noted that “the viral load of such asymptomatic persons has been equal to that of symptomatic persons, suggesting similar potential for viral transmission.”
One of the study’s author’s, Scripps Director Dr. Eric Topol, criticized the WHO’s comments on Twitter, writing that “there are several studies not included in [the WHO’s] brief statement that counter the scant data provided here.”
Carl Bergstrom, a biologist at the University of Washington, wrote on Twitter that the WHO’s conclusions were based on “thin evidence,” at least when taking into account what has been published publicly.
Bergstrom also said the organization should have more clearly distinguished between people who are “truly” asymptomatic—those who never show symptoms—and those who may unwittingly spread the disease in the days before they become symptomatic. Topol’s study on asymptomatic transmission found that few people who test positive without symptoms go on to develop them, but studies suggest it takes an average of five days after exposure to the virus for symptoms to surface. People in this phase would be considered pre-symptomatic, not asymptomatic, but it’s difficult to tell the difference.
“Even if truly asymptomatic spread is very rare, pre-symptomatic transmission is likely to be important,” Bergstrom wrote on Twitter. “We still need to wear masks and distance to avoid spreading the virus during this period, probably concentrated in days 3-6 after infection.”
Van Kerkhove acknowledged that distinction when speaking with TIME after the press briefing, and added that it can be difficult to distinguish between a mildly symptomatic and asymptomatic person. Some people may not associate mild symptoms—like fatigue or muscle aches—with COVID-19, but these individuals would still technically be symptomatic and capable of spreading the virus, Van Kerkhove says.
With so much uncertainty, Van Kerkhove says more research on transmission patterns and asymptomatic carriers is required. She says people should continue following public-health guidance such as wearing fabric face masks when social distancing is not possible, and should stay home if they feel unwell. Doing so, in conjunction with robust contact tracing and isolation of people with symptoms, will help keep COVID-19 spread under control, she says.
“We’re not ruling anything out,” Van Kerkhove says. “We’re not saying that [asymptomatic spread is] not happening. But we’re saying more transmission is happening among symptomatic individuals. People are looking for a binary, and it’s not that.”
Bergstrom was more direct. The WHO’s statement “seems to suggest that people without symptoms don’t spread COVID19,” Bergstrom tweeted. “Does this mean shoppers, students, protesters, etc., don’t need masks/ distancing? No.”