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.”