Asymptomatic spread of coronavirus is ‘very rare’, WHO says

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A World Health Organization (WHO) official said late on Monday that people with the Sars-Cov-2 may not be infectious if they are ‘asymptomatic’, a significant new claim about the understanding of the coronavirus disease (Covid-19) that experts around the world said could be premature.

WHO official epidemiologist Maria Van Kerkhove made the statement at a briefing in Geneva, where officials from the global health agency said that new coronavirus cases had their biggest daily increase ever and countries must continue with efforts to contain the virus.

“From the data we have, it still seems to be rare that an asymptomatic person actually transmits onward to a secondary individual,” said Van Kerkhove, head of WHO’s emerging diseases and zoonosis unit, said at a news briefing from the United Nations agency’s Geneva headquarters. “It’s very rare.”

These asymptomatic cases, she said, were identified through contact tracing of known patients and the lack of onward transmission by asymptomatic individuals was based on data from countries carrying out detailed contact tracing. “Much of that has been published in the literature yet,” she said, adding that the agency was reviewing such data to be sure.

Van Kerkhove said such findings were published in a paper from Singapore, where contacts from long-term care facilities and households supported the theory.

Last week, China announced results from its population-wide testing of residents in Wuhan, the epicenter of the outbreak, where they found 300 asymptomatic patients among the city’s 9.98 million residents.

Personal belongings of these asymptomatic carriers such as toothbrushes, mugs, masks and towels have shown negative for the virus in samples collected from the surfaces, Chinese news agency Xinhua reported on Tuesday. A total of 1,174 close contacts of the 300 cases have also been tested negative for coronavirus, the report added.

Experts have sought to ask the WHO to make a distinction between whether the lack of transmission was true only in asymptomatic cases – people with an infection that is so mild that they never have any symptoms – compared to pre-symptomatic cases where people go on to later develop symptoms.

In a study published in the journal Nature in mid-April, researchers from China’s Guangzhou studied 94 patients and estimated that 44% of secondary cases were infected during the index patient’s presymptomatic stage, particularly in settings such as households.

“Disease control measures should be adjusted to account for probable substantial presymptomatic transmission,” they added.