World Health Organization expert Maria van Kerkhove says people with coronavirus are most infectious just at the point when they first begin to feel unwell, day after remarks that transmission of virus by people with no symptoms is "very rare."
A top World Health Organization (WHO) expert has tried to clear up "misunderstandings" about comments she made that were widely understood to suggest that people without Covid-19 symptoms rarely transmit the coronavirus.
Maria Van Kerkhove, the UN health agency's technical lead on the virus pandemic, insisted on Tuesday that she was referring only to a few studies, not a complete picture, in the comments she made on Monday.
Those remarks raised confusion and questions among outside experts and health officials who have recommended and in some places required that people wear masks to try to prevent the virus from spreading.
'Subset of studies'
The "clarification" she provided during a WHO social-media chat showed many questions remain about whether infected people who don't show symptoms of illness such as fever, dry cough or difficulty breathing can transmit the virus to others.
Van Kerkhove said, "What I was referring to yesterday were very few studies, some two or three studies that have been published, that actually try to follow asymptomatic cases."
"That’s a very small subset of studies," she continued. "I used the phrase 'very rare, and I think that that’s (a) misunderstanding to state that asymptomatic transmission globally is very rare. What I was referring to was a subset of studies."
Experts befuddled by previous remarks
"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 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.
Study says one in five of infected were symptomless
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.