Spread by people without symptoms is much worse than first thought

This scanning electron microscope image shows SARS-CoV-2 (orange)—also known as 2019-nCoV, the virus that causes COVID-19—isolated from a patient in the U.S., emerging from the surface of cells (green) cultured in the lab. Credit: NIAID-RML

When the history of this coronavirus pandemic is written, the initial failure by health authorities to understand the danger of transmission by people not showing symptoms will be seen as a major factor in its spread.

The information about “asymptomatic” carriers — those spreading the virus without showing symptoms — remains a work in progress and estimates of numbers are all over the map.

Enough is now known to underscore the message that strict social distancing is still very necessary, even when people aren’t showing symptoms.

China has been warning about asymptomatic transmission since January, but there was initial resistance to this idea by Western health authorities.

When she testified to the House of Commons Health Committee at the end of January and early in February, Dr. Theresa Tam, Chief Public Health Officer of Canada, responded to questions from MPs by downplaying the threat of asymptomatic transmission.

“Based on what we know about those coronaviruses, is it possible that an asymptomatic person could transmit the virus? Even if it’s possible, it is, we believe, a rare event. It is not that type of transmission that drives the force of an epidemic,” Tam said at a hearing on January 29.

That initial skepticism probably played a role in the virus gaining a foothold in Canada as officials scrambled to repatriate Canadians from China, and attention was focused on those showing symptoms.

Tam, who is highly respected, was certainly not alone in underestimating the threat of asymptomatic transmission. She deferred to the World Health Organization (WHO), which was also grappling with this question.

Far from being a “rare event,” classified Chinese data now suggests as many as a third of transmissions were from “silent carriers,” as reported earlier this week by the South China Morning Post.

Earlier estimates by WHO were that only one to three percent of cases resulted from asymptomatic infection, the South China Morning Post said.

Rare among Western health authorities has been Dr. Deborah Birx, the Trump administration’s coronavirus response coordinator, who has expressed concerns that asymptomatic transmission may be a bigger concern.

This recent CNN story outlines some of the research that supports this concern.

A group of Japanese experts led by Hiroshi Nishiura, an epidemiologist at Hokkaido University, wrote in a letter to the International Journal of Infectious Diseases in February. Nishiura’s researchers estimated that the proportion of asymptomatic Japanese patients evacuated from Wuhan, China, was 30.8 per cent — roughly consistent with the Chinese information.

South Korea, which leads the world in its public testing for the virus, has also found large numbers of asymptomatic carriers. As of Friday, March 20, South Korea had done nearly 317,000 tests in a country with a population of just over 50 million people.

China and South Korea, two countries that engaged aggressive public testing, are the only two countries that have seen significant declines in the number of new cases.

A research letter by American and Chinese researchers posted by the U.S. Centers for Disease Control and Prevention (CDC), found that the mean “serial interval” for Covid-19 is 3.96 days, based on data from Chinese cases up to early February.

“The serial interval of COVID-19 is defined as the time duration between a primary case-patient (infector) having symptom onset and a secondary case-patient (infectee) having symptom onset,” the research letter explains.

They also found that 12.6 percent of case reports indicated presymptomatic transmission.

See also this report in Science Daily.

“This provides evidence that extensive control measures including isolation, quarantine, school closures, travel restrictions and cancellation of mass gatherings may be warranted,” said Lauren Ancel Meyers, a professor of integrative biology at University of Texas Austin. “Asymptomatic transmission definitely makes containment more difficult.”

Italy has been ravaged by the coronavirus pandemic, but one bright spot has been an experiment in Vò, a town of about 3,000 people west of Venice.

In Vò, broad testing of the population was carried out along with contact tracing. The town was able to eliminate the virus from its population.

This story in the UK newspaper, The Guardian, describes the experiment in Vò in greater detail.

“Nonetheless, asymptomatic or quasi-symptomatic subjects represent a good 70 per cent of all virus-infected people and, still worse, an unknown, yet impossible to ignore portion of them can transmit the virus to others. Full testing would give us a clearer picture of how many people actually have the virus, and how many pass it on,” wrote Andrea Crisanti, a professor of microbiology at the University of Padua, and Antonio Cassone a former director of the department of infectious diseases at the Italian institute of health.

While large-scale public testing is the only way to truly get a handle on the number of asymptomatic cases spreading the virus, a severe shortage of test kits in North America has precluded this option.

Here in B.C., authorities are focusing testing on healthcare workers and cases in hospitals in an effort to keep the health system safe. With limited testing capacity, this probably makes the most sense.

But widespread testing, when it becomes available, will be a valuable tool in controlling asymptomatic spread.

Given what we now know about asymptomatic transmission, and are learning each day, it’s wise to ignore earlier assurances that it “is not that type of transmission that drives the force of an epidemic.” It’s best to assume that others — whether or not they show symptoms — may be infectious.

This graph shows Covid-19 tests by country as of March 20. Canadian data is not agglomerated. (Source: OurWorldInData.org/coronavirus)

Vò, Italy

Author: Richard McGuire

Richard McGuire is an Osoyoos photographer who worked at the Osoyoos Times between 2012 and 2018, first as reporter and then as editor. He has a long career in journalism as well as research, communication and management at the House of Commons in Ottawa and in the federal government.