The week of coronavirus declarations and clarifications by the World Health Organization
Researcher backtracks on statement that transmission of COVID-19 by people with no symptoms is ‘very rare’ • Most patients most infectious when they first feel unwell
GENEVA (Reuters) – Studies show people with the coronavirus are most infectious just at the point when they first begin to feel unwell, World Health Organization experts said last week.
This feature has made it so hard to control spread of the virus that causes COVID-19 disease, but it can be done through rigorous testing and social distancing, they said.
“It appears from very limited information we have right now that people have more virus in their body at or around the time that they develop symptoms, so very early on,” Maria Van Kerkhove, a WHO epidemiologist and technical lead on the pandemic, told a live session on social media.
Preliminary studies from Germany and the United States suggest that people with mild symptoms can be infectious for up to 8-9 days, and “it can be a lot longer for people who are more severely ill,” she said.
Earlier, some disease experts questioned her statement on Monday 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.
Van Kerkhove, citing disease-modeling studies, clarified on Tuesday that some people do not develop symptoms but can still infect others.
“Some estimates of around 40% of transmission may be due to asymptomatic [cases], but those are from models. So I didn’t include that in my answer yesterday, but wanted to make sure that I made that clear,” she said.
WHO Health Emergencies Program executive director Michael Ryan said that the novel coronavirus lodges in the upper respiratory tract, making it easier to transmit by droplets than related viruses such as SARS or MERS, which are in the lower tract.
“Now as we look at COVID-19, we have an infectious pathogen that is present in the upper airway for which the viral loads are peaking at the time you are just beginning to get sick,” he said.
“That means you could be in the restaurant, feeling perfectly well, and start to get a fever; you are feeling ok, you didn’t think to stay home, but that’s the moment at which your viral load could be actually quite high,” he said.
Ryan added: “And it’s because the disease can spread at that moment that the disease is so contagious; that’s why it spread around the world in such an uncontained way, is because it’s hard to stop this virus.”
But some countries have shown that transmission can be brought down to “an acceptable level or even to no level,” as New Zealand had recently demonstrated, he said.
VAN KERKHOVE 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.
“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 presymptomatic 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 to 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 are sparse on how likely such people are to transmit the disease.
The cohead of Singapore’s coronavirus task force said on Monday that 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 epicenter, 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.
The WHO’s Van Kerkhove clarified comments she made the day before that asymptomatic transmission of the coronavirus is “very rare,” saying her comments were in reference to “two or three studies” following asymptomatic individuals.
She said that her remarks were not in reference to the potential global spread of asymptomatic individuals, and underscored that experts still lack sufficient data on how many people without symptoms are infected. She also highlighted modeling from various groups that estimate asymptomatic transmission could be as high as 40% of all transmissions.
“What we need to better understand is how many people don’t have symptoms, and separately, how many of those individuals transmit to others,” she said.
Because the virus is transmitted through droplets, people with symptoms like coughing are generally understood to be at a higher risk of spreading the virus. But asymptomatic people could potentially spread the virus through yelling or singing, the WHO’s Ryan said Tuesday.
The clarification did not fully placate critics of Van Kerkhove’s remarks.
“[P]ublic health communication isn’t ancillary to public health,” former Centers for Medicare and Medicaid Services acting administrator Andy Slavitt said in a lengthy thread on Twitter. “It is the central component in battling it.”