WHO Says Symptomless Spread Is ‘Rare,’ in Jolt to Virus Efforts

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Transmission of the novel coronavirus by people who aren’t showing symptoms is “very rare,” theWorld Health Organization said, contradicting awidely held belief among health officials and researchers that the disease was being spread by people who weren’t showing signs of illness.

“It still appears to be rare that an asymptomatic person actually transmits onward to a secondary individual,” Maria Van Kerkhove, head of WHO’s emerging diseases and zoonosis unit, said at a briefing in Geneva. She said her comment is based on detailed reports of contact tracing from various countries.

More research is needed to confirm the roles that so-called asymptomatic cases and pre-symptomatic cases play in spreading the disease. But if proven correct, the development could have a major impact on how health officials and governments approach the battle to contain the pandemic. Uncertainty over how the virus behaves has hindered nations’ efforts to re-open their battered economies.

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Earlier research sparked concern that the virus would be difficult to contain because of asymptomatic transmission. The New England Journal of Medicine, in an article dated May 28, warned that transmission of SARS-CoV-2 by seemingly healthy people is “the Achilles’ heel of Covid-19 pandemic control.”

Rigid Restrictions

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.

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 thetesting of its entire population of 11 million in an effort to identify cases to avoid a resurgence of infections.

Van Kerkhove cited a number of reports from countries doing detailed contact tracing -- in which asymptomatic cases and their contacts were followed -- that found no evidence of secondary transmission. She said countries should focus on following symptomatic cases.

“If we actually follow all the symptomatic cases, isolated those cases, followed the contacts and quarantined those contacts, we would drastically reduce” transmission, she said.

— With assistance by Corinne Gretler

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