Update: Late on June 9, WHO official Dr Maria Van Kerkhove revised her statement that asymptomatic people pose very little infection risk, saying it was based on a small number of studies, and unpublished data.
The spread of Covid-19 by people who never show symptoms is “very rare”, according to a World Health Organisation official, casting doubt over the widely-held belief that such people are key drivers of the pandemic.
Dr Maria Van Kerkhove, head of the WHO’s emerging diseases and zoonosis unit, said new data from nations with comprehensive tracing programmes indicated that such “asymptomatic” people rarely pass on the virus to others.
“The available evidence from contact tracing reported by Member States suggests that asymptomatically-infected individuals are much less likely to transmit the virus than those who develop symptoms”, she said. “From the data we have, it still seems to be rare that an asymptomatic person actually transmits onward to a secondary individual”.
This contradicts the widely-held view that such people pose the greatest risk to the public are asymptomatic spreaders, by unwittingly infecting others without realising they have the virus. Some may also become "super spreaders", driving the pandemic by infecting many people through their work or travel patterns.
Substantial numbers of people have passed on the virus prior to them developing symptoms so I don't think we can say overall that asymptomatic spread is very rare
Prof Liam Smeeth, London School of Hygiene and Tropical Medicine
However, the WHO statement suggest asymptomatic people pose little risk to others. One possible explanation is that they defeat the infection quickly and never develop the classic symptom of a dry, continuous cough which spreads the virus.
Even so, Monday's announcement, made during a news briefing at the WHO headquarters in Geneva, has surprised many experts.
Prof Liam Smeeth, professor of clinical epidemiology at the London School of Hygiene and Tropical Medicine, told the BBC's Today programme it was more likely people were spreading the virus after immediately contracting it.
“Substantial numbers of people have passed on the virus prior to them developing symptoms so I don’t think we can say overall that asymptomatic spread is very rare,” he said.
Prof Carl Bergstrom, theoretical biologist at the University of Washington, Seattle, said that the WHO findings would be an "absolute game changer" for modelling work on prevention strategies.
If correct, the WHO statement suggest more infections are being caused by "pre-symptomatic" people, who are in the earliest stage of Covid-19 infection and – unlike asymptomatics – do go on to develop symptoms.
That would have major implications for current testing-trace-isolate programmes, according to Prof Bergstrom.
“When asymptomatic people transmit disease you have a broad window across which it helps to catch them via testing”, he said. “When only pre-symptomatic people transmit disease, you only have a couple of days to catch them before symptoms develop”.
He said this would require far more frequent – perhaps even daily – testing of people.
Other researchers questioned the basis of the WHO’s statement.
Prof Eric Topol of the Scripps Research Institute, San Diego, and co-author of a review of the latest evidence on the role of asymptomatic people in the pandemic, said there was insufficient data to make that assertion.
According to Prof Topol, his review suggests that asymptomatic people may be responsible for around 40 – 45 per cent of infections, and can transmit the virus for perhaps as long as 14 days.
Published in the current Annals of Internal Medicine, the review suggests that, "because of the high risk for silent spread of asymptomatic persons, it is imperative that testing programmes include those without symptoms".
This contradicts the WHO’s new advice that governments should focus on detecting infected people showing symptoms.
“What we really want to be focused on is following the symptomatic cases”, said Dr Van Kerkhove.
She conceded there is still some uncertainty about the role of asymptomatic people in the pandemic, adding that the WHO continues to monitor reports.
"We are constantly looking at this data and trying to get more information from countries to truly answer this question," she said.
Robert Matthews is Visiting Professor of Science at Aston University, Birmingham, UK