Asymptomatic carriers of coronavirus rarely spread virus, WHO expert says

WHO expert’s findings on carriers has stirred controversy with the research community

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After months of medical experts warning that coronavirus carriers with or without symptoms could transmit the virus, an expert at the World Health Organization on Monday brought into question that theory for those without symptoms.

Maria Van Kerkhove, WHO’s technical lead on COVID-19, said at a news briefing Monday that although the asymptomatic COVID-19 patients still have potential to spread the virus it is “rare,” with most of the transmission happening from those who show the litany of symptoms associated with the virus.  Many countries are reporting cases of spread from people who are asymptomatic, including Britain and the U.S. However, Van Kerkhove explained the spread of the virus from those without symptoms makes up only about 6% of the spread − at most.

“We are constantly looking at this data, and we’re trying to get more information from countries to truly answer this question,” she said. “It still appears to be rare that asymptomatic individuals actually transmit onward.”

Numerous studies have suggested the virus is spreading from people without symptoms, but many of those are either anecdotal reports or based on modeling.

Van Kerkhove said that based on data from countries, when people with no symptoms of COVID-19 are tracked over a long period to see if they spread the disease, there are few cases of spread. In an interview with TIME following the presser, Van Kerkhove clarified the findings do not mean it's impossible for asymptomatic patients to spread the virus.

“I did not say that asymptomatic cases cannot transmit; they can,” Van Kerkhove said. “The question is, do they? And if they do, how often is that happening?”

WHO has not released data that explicitly states Van Kerkhove’s findings. In its latest guidance, WHO offered more information about how and when to wear masks. Future findings will denote more on the COVID-19 community spread from yet-to-be-published contact tracing reports in a number of states.

Last week, researchers from the Scripps Research Translational Institute published a paper estimating that asymptomatic coronavirus patients account for up to 45% of 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.”

Scripps Director Dr. Eric Topol, one of the study’s authors, 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.”

Van Kerkhove revealed to TIME that it may be hard to pinpoint numbers because some will consider themselves asymptomatic despite showing signs including fatigue or muscle aches. Those symptoms would still be considered symptomatic, she said.

“We’re not ruling anything out,” Van Kerkhove said. “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.”