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The World Health Organization said it’s still unclear how readily the novel coronavirus is spread by people who don’t develop symptoms, a day after a top official sparked debate by saying such transmission is “very rare.”

Maria Van Kerkhove, head of WHO’s emerging diseases and zoonosis unit, said Tuesday that her previous comment had been misunderstood, since she was referring only to certain studies on the spread of COVID-19 involving contact tracing.

Although the health organization had said as far back as February that it did not see asymptomatic cases as a major cause of viral spread, Van Kerkhove’s remark at a news conference Monday revived controversy over coronavirus transmission routes. Uncertainty over the issue has hindered nations’ efforts to re-open battered economies, with the New England Journal of Medicine previously warning that transmission by seemingly healthy people is “the Achilles’ heel of COVID-19 pandemic control.”

Some of the confusion lies in the distinction between the roles played by truly asymptomatic people and those who are merely pre-symptomatic — and later go on to become ill — in spreading the disease.

“It still appears to be rare that an asymptomatic person actually transmits onward to a secondary individual,” Van Kerkhove said Monday.

Pre-symptomatic individuals, who develop a higher viral load just before the onset of symptoms, may be infectious, the WHO said in guidance on the use of face masks that it issued last week. The contagion is spread by tiny droplets expelled when infected people sneeze, cough, speak or breathe.

“Comprehensive studies on transmission from asymptomatic individuals are difficult to conduct, but the available evidence from contact tracing reported by member states suggests that asymptomatically infected individuals are much less likely to transmit the virus than those who develop symptoms,” the group said then.

Van Kerkhove’s remarks Monday sparked new debate among health experts.

“Contrary to what the WHO announced, it is not scientifically possible to affirm that asymptomatic carriers of SARS-CoV-2 are not very infectious,” professor Gilbert Deray of the Pitie-Salpetriere hospital in Paris said on Twitter.

Liam Smeeth, a clinical epidemiology professor at the London School of Hygiene and Tropical Medicine, said he was “quite surprised.”

“There remains scientific uncertainty, but asymptomatic infection could be around 30 percent to 50 percent of cases. The best scientific studies to date suggest that up to half of cases became infected from asymptomatic or pre-symptomatic people,” he said.

Van Kerkhove attempted to clarify her remarks Tuesday in a live event on social media.

“I used the phrase ‘very rare’ and I think that’s a misunderstanding to state asymptomatic transmission globally is very rare,” she said.

While it’s known that some asymptomatic patients can transmit the virus, how many there are and how many are contagious needs further study, Van Kerkhove said. Asymptomatic people tend to be younger and without underlying medical conditions, she said.

Countries across the globe have been wary of relaxing social-distancing guidelines and rigid travel restrictions, fearing that people without symptoms could spread the COVID-19 pathogen unchecked throughout communities.

“The asymptomatics are still important, particularly if you want to get levels of virus down to very low levels of transmission,” said Peter Collignon, a professor of clinical medicine at the Australian National University Medical School in Canberra, who advises the Australian government on infection control.

Because identifying asymptomatic cases is so difficult, the U.S. and other nations have struggled to implement adequate testing to gauge how widespread the disease has become. The Chinese city of Wuhan recently completed the testing of its entire population of 11 million in an effort to identify cases to avoid a resurgence of infections.

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