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Home >Science >Health >WHO says 40% of Covid-19 transmissions may be through asymptomatic people

WHO says 40% of Covid-19 transmissions may be through asymptomatic people

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Maria van Kerkhove, Technical Lead of the World Health Organization (Photo: Reuters)

  • Wearing a mask reduces the possibility of asymptomatic transmission or 'pre-symptomatic' transmission, says WHO's Maria van Kerkhove
  • There is a period just as a person is becoming unwell when the virus spreads, that is what makes it so hard to stop, says WHO's Michael Ryan

Geneva: Studies show people with the coronavirus are most infectious when they first show symptoms, Maria van Kerkhove, an epidemiologist and the World Health Organization's technical lead on the pandemic, said in a briefing on Tuesday.

Geneva: Studies show people with the coronavirus are most infectious when they first show symptoms, Maria van Kerkhove, an epidemiologist and the World Health Organization's technical lead on the pandemic, said in a briefing on Tuesday.

She said a sub-set of people do not develop symptoms, but can still infect others, and as many as 40% of transmissions may be by asymptomatic cases. Wearing a mask reduces the possibility of asymptomatic transmission or 'pre-symptomatic' transmission, she said.

She said a sub-set of people do not develop symptoms, but can still infect others, and as many as 40% of transmissions may be by asymptomatic cases. Wearing a mask reduces the possibility of asymptomatic transmission or 'pre-symptomatic' transmission, she said.

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"Need to actively pursue research studies to fill in gaps on how virus spreads. Question is not only who is transmitting to others but when and data is very preliminary on it. Studies show people are more infectious at or around the time they develops symptoms, so very early on," Kerkhove added.

WHO Health Emergencies Programme Executive Director Michael Ryan said, "Question remains how does someone who is asymptomatic spread coronavirus, studies show it can be projected by people in choirs, gyms."

Ryan said lockdowns had great impact in stopping transmission and that we'll see "small spikes" in cases and have to make they don't turn into big new peaks.

"There is a period just as a person is becoming unwell when the virus spreads, that is what makes it so hard to stop," said Ryan.

Ryan said he's "absolutely convinced" that transmission by asymptomatic cases is occurring.

Kerkhove attempted to downplay her comment Monday that asymptomatic transmission of coronavirus is “very rare", an assessment that revived controversy over transmission routes.

In a live event on social media to take questions from the public on Tuesday, Kerkhove said her comments referred to two to three studies that have been published and that try to follow asymptomatic cases over time and look at all their contacts to see how many additional people were infected.“That’s a very small subset of studies," said Van Kerkhove, a WHO epidemiologist.

“I was responding to a question, not stating a policy of the WHO. I was just trying to articulate what we know. I used the phrase ‘very rare’ and I think that’s a misunderstanding to state asymptomatic transmission globally is very rare. What I was referring to was a subset of studies. I also referred to some data that isn’t published."Van Kerkhove said that while it’s known there are some asymptomatic patients who can transmit the virus, it needs to be better understood how many of the people in the population don’t have symptoms and separately, how many of those individuals go on to transmit to others. Van Kerkhove also said asymptomatic patients tend to be younger and without underlying medical conditions.

Her comments elicited criticism from some public health experts.

“If you look at the data overall, I think it’s overwhelming that there has to be asymptomatic carriers that are spreading," said Mark Dybul, the co-director of Georgetown University’s Center for Global Health Practice and Impact. “It’s almost impossible that they don’t. I’d have to look at why she’s saying that, because it runs counter to everything, the growing body of evidence over the last five months."

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.

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.

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