WHO clarifies evolving assessment of asymptomatic COVID-19 spread

Health worker with face shield in Brazil
Health worker with face shield in Brazil

Agência Senado, Leopoldo Silva / Flickr cc

World Health Organization (WHO) officials today clarified comments from a media briefing yesterday that portrayed asymptomatic spread of COVID-19 as rare, triggering concerns among epidemiologists and health providers about the accuracy of the information and unclear messaging for the public.

The global COVID-19 total today rose to 7,185,573 cases, and 408,954 people have died from their infections, according to the Johns Hopkins online dashboard.

Knowns, unknowns on asymptomatic spread

Two WHO officials addressed questions about yesterday's briefing comments during a Facebook live session. The comments came yesterday after a reporter asked about asymptotic spread related to a media report from Singapore that said a number of asymptomatic cases were identified during recent large-scale testing.

During yesterday's briefing, Maria Van Kerkhove, PhD, the WHO's technical lead for COVID-19, said countries were finding asymptotic cases during contact tracing and that, based on unpublished data, secondary spread from people who have no symptoms appears to be very rare. However, the group has said transmission from asymptomatic and presymptomatic people does occur, a factor that underpins distancing recommendations and face mask advice.

In the wake of yesterday's comments, several scientists and health providers questioned the WHO's interpretation and information sources and worried that the comments added confusion to crucial public health messaging, especially in light of mask recommendations.

During today's Facebook live session, Van Kerkhove and Mike Ryan, MD, who directs the WHO's health emergencies program, clarified comments and answered questions. Van Kerkhove said that although most transmission involves sick people passing infectious droplets, there is an undetermined subset of people who don't ever have symptoms who pass the virus to others.

"There are some estimates that anywhere from 6% of the population to 40% of the population may be infected but not have symptoms, with a point estimate of around 16%," she said.

She said that, in answering yesterday's media question, she wasn't laying out WHO's policy, but was sharing what the agency currently knows from a variety of different sources, including two or three studies that followed asymptomatic people to see how many contacts they infected. "And that's a very small subset of studies."

The use of the "very rare" phrase was a misunderstanding that wasn't meant to portray that asymptomatic transmission globally is very rare, but rather applies to the small subset of studies, Van Kerkhove said.

In fielding questions today about the WHO's data sources, she said that, along with published studies, the WHO continually receives information presented by experts and member countries, who share the latest information from, for example, their cluster investigation and contact tracing experiences, which includes what they are seeing regarding COVID-19 transmission.

She said the level of spread from asymptomatic people is a major unknown, and that what she didn't mention in yesterday's media briefing was that some modeling groups have tried to estimate the proportion, and there's a big range, with some groups estimating around 40% of transmission may be due to asymptomatic cases.

Another big question is what proportion of people are truly asymptomatic and never progress to having any symptoms and what proportion reported as asymptomatic actually have mild disease. "We're 6 months into a pandemic, there's a huge amount of research that's being done, but we don't have that full picture yet," Van Kerkhove said.

The second big question is what proportion of those who don't have symptoms actually transmit, she added. "And that's a big open question, and it remains an open question."

What is known is that studies have found that patients shed virus 1 to 3 days before their symptoms begin, Van Kerhhove said, though a better understanding is still needed about what proportion of that is contributing to transmission.

Increases in India, Indonesia, Africa

In other international developments, the deputy minister of India's capital, New Delhi, today warned that the city's cases may surge from 29,000 to 550,000 by the end of July, amid reports that people are already struggling to get hospital care, Reuters reported.

Manish Sisodia said by late July, the city will need 80,000 hospital beds, and it has a current capacity of 9,000. With 266,958 cases, India has the world's fifth highest total.

Meanwhile, Indonesia today reported 1,043 cases, a new daily high, as the country resumed domestic flights, according to Reuters. Safety rules limit airlines to operate at 70% capacity and for passengers to wear masks, observe physical distancing, and show a certificate that they tested negative for COVID-19.

In Africa, COVID-19 activity continues to grow, with a 30% increase in confirmed cases in the last reporting week, the WHO's African regional office said today in its weekly outbreaks and health emergencies report. The agency said the region reported its highest daily total on Jun 4, led by cases in South Africa, some of which involves the country's backlog in reporting.

With cases rising steadily, many countries are reporting newly affected geographic areas, including smaller urban areas and rural communities. Three countries—Cabo Verde, Mauritania, and Uganda—reported their first healthcare worker infections.

South Africa, Nigeria, Algeria, Ghana, and Cameroon have reported the most cases. Meanwhile, the countries seeing the largest rise in cases over the past week include Mauritania, Ethiopia, Mozambique, Uganda, Central African Republic, Zimbabwe, and Malawi.

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