WHO backtracks after saying transmission from people who don’t show coronavirus symptoms is 'very rare'

WHO clarified that Covid-19 can be spread by people without symptoms, stating that how much coronavirus transmission comes from asymptomatic people is still a big unknown


                            WHO backtracks after saying transmission from people who don’t show coronavirus symptoms is 'very rare'
(Getty Images)

A top expert at the World Health Organization (WHO) has walked back from a previous assertion that transmission of the new coronavirus by people who did not have symptoms was “very rare.” The backtracking came after severe criticism by scientists who said multiple studies have shown transmission of Covid-19 from asymptomatic people. The organization’s Covid-19 technical lead, Dr Maria Van Kerkhove, who had made the comment, acknowledged that some modeling studies estimate that up to 40% of coronavirus infections could be transmitted by people who have the virus but no symptoms.

What did the WHO say initially?

On Monday (June 8), at a WHO briefing, Dr Kerkhove said that from the data they have it seems to be rare that an asymptomatic person actually transmits onwards to a secondary individual. “We have a number of reports from countries who are doing very detailed contact tracing, they are following asymptomatic cases, they are following contacts and they are not finding secondary transmission onwards. It's very rare. And much of that is not published in the literature,” she said. “We are constantly looking at data. We are trying to get more information from countries to truly answer this question (on what proportion of asymptomatic people transmit). It still appears to be rare that an asymptomatic individual actually transmits onwards,” she added.

Dr Kerkhove said that what countries need to focus on is following the symptomatic cases. “If we followed all of the symptomatic cases, isolated those cases, followed the contact, and quarantined those contacts, we would drastically reduce transmission,” she said. Dr Kerkhove also tweeted lines from a WHO summary on the transmission of Covid-19: “Comprehensive studies on transmission from asymptomatic individuals are difficult to conduct, but 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.” 

The clarification followed

The WHO typically schedules Covid-19 briefings on Mondays, Wednesdays, and Fridays. The organization was, however, forced to clarify its position on Tuesday on whether people without symptoms are spreading the new coronavirus, stressing that much remains unknown about asymptomatic transmission and that they do not have the answer yet on the matter.

Stating that some asymptomatic people can transmit the virus on, Dr Kerkhove said what experts need to better understand is how many of the people in a population do not have symptoms and separately how many of those individuals go on to transmit to others. “I was responding to a question at the press conference. I was not stating a policy of the WHO or anything like that. I was just trying to articulate what we know. In that, I used the phrase ‘very rare.’ And I think that's a misunderstanding to state that asymptomatic transmission globally is very rare,” she said.

“What we do know about transmission is that people who are infected with Covid-19 many develop symptoms but there are some people who do not. The majority of transmission that we know about is that people who have symptoms transmit the virus to other people through infectious droplets. But there is a subset of people who don't develop symptoms and to truly understand how many people don't develop symptoms we don't actually have that answer yet. There are some estimates that suggest that anywhere between 6% of the population and 41% of the population may be infected but don't have symptoms, within a point estimate of around 16%,” said Dr Kerkhove. 

The WHO's Covid-19 technical lead, Dr Maria Van Kerkhove, said it was a misunderstanding to state that asymptomatic transmission globally is very rare (Getty Images)

Dr Kerkhove clarified that her comment at the WHO’s Monday press briefing about its rarity was based on two or three studies that follow asymptomatic cases and their contacts to understand how many additional people were infected, as well as unpublished data shared by countries or experts with the organization. “What I was referring to was a small subset of studies. I also referred to some data that isn't published. And this is information that we received from our member states, through either presentation that they give at member state briefings or presentations that are given to us through teleconferences,” she said.

She explained further, “We convene global expert networks. and at many of those, we discuss ongoing research ongoing studies that are there. I was referring to some detailed investigations, cluster investigations, case-contract tracing where we had reports from member states saying that 'when we follow asymptomatic cases, it's very rare that we found a secondary transmission'.”

Dr Kerkhove further explained that what she did not report at the Monday briefing was because this is a major unknown, some modeling groups have tried to estimate what is the proportion of asymptomatic people that may transmit. “These are estimates. And there is a big range from the different models depending on how the models are done, from which country. But some estimates of around 40% of transmissions maybe due to asymptomatic. But those are from models. So I didn't include that in my answer yesterday (Monday),” she said.

What did scientists have to say?

Experts say while asymptomatic transmission does occur, no one knows for sure how frequently it happens, and this takes time to study. Accordingly, Dr Kerkhove’s comments on Monday sparked off a furious scientific debate, attracting widespread criticism of WHO’s public health messaging. The original comments incited strong pushback from scientists and public health experts, who suggested the agency had made a mistake or at least miscommunicated when it said people who did not show symptoms were unlikely to spread the virus. They said more data are needed before WHO or anyone else can declare asymptomatic transmission rare.

Dr Carl T Bergstrom, professor of Biology at University of Washington, tweeted: “What we've ended up with here is public health by press release (making recommendations without providing the underlying data and analysis) coupled with poor communications (failing to clearly explain that they are not talking about pre-symptomatic transmission) on WHO’s part.”

Dr A Marm Kilpatrick, professor at the Department of Ecology & Evolutionary Biology, University of California, Los Angeles, said, “Sadly it's not the 1st time WHO is putting out poor information... Given the uncertainty in infectiousness and bias in contact tracing methodology, there is insufficient evidence for WHO to make this claim for asymptomatic transmission.” 

Dr Ashish Jha, professor of global health at the Harvard TH Chan School of Public Health and the faculty director of the Harvard Global Health Institute, said: “About 20% of people who are infected likely never develop any symptoms...Some modeling studies suggest 40-60% of spread is from people when they didn’t have symptoms. Both asymptomatic and pre-symptomatic spread (a) huge problem for controlling the disease. Because folks (are) shedding virus while asymptomatic. Bottom line question: Are infected people without symptoms an important cause of spread? My best guess: yes.” He said: “People without symptoms definitely spread disease (so wear a mask). WHO should be clearer in communication.” 

Experts said that people without symptoms can and do spread Covid-19, so people must continue to wear masks when around others (Getty Images)

The Harvard Global Health Institute also issued a statement saying that the WHO created confusion when it reported on Monday that asymptomatic patients rarely spread the disease. “All of the best evidence suggests that people without symptoms can and do readily spread SARS-CoV2, the virus that causes Covid-19. The government needs to continue to ramp up testing to focus on testing for both symptomatic and asymptomatic patients to keep America safe,” says the statement. 

The experts said that implications here are considerable because people without symptoms “can and do spread the disease” and people must continue to wear masks when around others, and maintain a six-feet distance. “Communicating preliminary data about key aspects of the coronavirus without much context can have a tremendous negative impact on how the public and policymakers respond to the pandemic. If new evidence becomes available, WHO should be transparent, make all available data publicly accessible and take the time to thoroughly brief the media and the public on the nature and interpretations of the findings.”

Several studies show that both asymptomatic and presymptomatic people can spread the virus. A recent study said that asymptomatic persons seem to account for approximately 40% to 45% of Covid-19 infections, and they can transmit the virus to others for an extended period, perhaps longer than 14 days. Co-author of this study and director and founder of the Scripps Research Translational Institute, Dr Eric Topol, said, “To provide meaningful data on the magnitude of transmissibility from asymptomatic people, a prospective study would need to be done to quantify spread and ideally include contact tracing, forward and backward, culture of the virus, and virus genomic sequence to precisely define the route.” 

Dr Natalie E Dean, assistant professor of biostatistics at the University of Florida, said the takeaway from the whole asymptomatic/pre-symptomatic debate is that there is evidence that people can transmit to others before they develop symptoms, implying that people without current symptoms can be infectious. “This is observed by tracing data. There are instances where someone was infected but their only possible exposure was a pre-symptomatic contact. To be sure, we must rule out all other potential sources of infection (fomites, untraced contacts),” she said. She explained that experts still do not know how often asymptomatic people transmit to others, mostly because they are so difficult to find, and even more difficult to link in chains of transmission. “This is a new virus. There is still a lot of uncertainty. But we know that people without symptoms can transmit, and so our policies and actions must reflect this,” said Dr Dean.

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