World Health Organization officials are walking back some statements from Monday, now saying that they are “absolutely convinced” that asymptomatic transmission of the novel coronavirus is occurring.
Disease experts on Tuesday questioned a statement by the World Health Organization that transmission of COVID-19 by people with no symptoms is “very rare,” saying this guidance could pose problems for governments as they seek to lift lockdowns.
Maria van Kerkhove, an epidemiologist and the WHO’s technical lead on the coronavirus pandemic, said on Monday that many countries undertaking contact tracing had identified asymptomatic cases, but were not finding they caused further spread of the virus. “It is very rare,” she said.
In a briefing on Tuesday, Van Kerkhove said that some models estimated that up to 40 per cent of coronavirus transmission might be due to spread by asymptomatic cases, something she did not mention in her Monday comments.
The science isn’t settled, though, and some important questions remain, she said. The first is, “How many people are infected with COVID-19 and really don’t develop symptoms?” she said.
“The second big question is what proportion of those that don’t have symptoms actually transmit?”
“I was quite surprised by the WHO statement,” said Liam Smeeth, a professor of clinical epidemiology at the London School of Hygiene and Tropical Medicine, who added that he had not seen the data van Kerkhove’s statement was based on.
“It goes against my impressions from the science so far that suggest asymptomatic people — who never get symptoms — and pre-symptomatic people are an important source of infection to others.”
Smeeth and other experts said understanding the risks of transmission among people with mild or no symptoms is crucial as governments begin to ease the lockdown measures they imposed to try and reduce the pandemic’s spread and gradually replace them with case tracking and isolation plans.
“This has important implications for the track/trace/isolate measures being instituted in many countries,” said Babak Javid, a Cambridge University Hospitals infectious diseases consultant.
Some experts say it is not uncommon for infected people to show no symptoms.
A non-peer-reviewed study from Germany in May based on 919 people in the district of Heinsberg — which had among the highest death tolls in Germany — found that about one in five of those infected were symptomless.
But data is sparse on how likely such people are to transmit the disease.
The co-head of Singapore’s coronavirus task force told Reuters on Monday there had been asymptomatic transmission cases there, between people living in close quarters.
China said last week that 300 symptomless COVID-19 carriers in its central city of Wuhan, the pandemic’s epicentre, had not been found to be infectious.
Keith Neal, a professor of the epidemiology of infectious diseases at Britain’s University of Nottingham, said that while the question of how big a role asymptomatic transmission plays in new infections is unclear, what is known is that people with symptoms are responsible for most of the spread of the disease.
“This reinforces the importance of any person who has any of the symptoms of COVID-19 arranges a test … as soon as possible and isolating until they get their test result,” he said.
Dr. Mike Ryan of the WHO said Tuesday that the organization is “absolutely convinced” that asymptomatic transmission is occurring and that wearing masks can help to cut the risk of passing the virus along.
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