Open this photo in gallery In this file photo taken on Feb. 27, 2020, this handout illustration image courtesy of the National Institutes of Health taken with a transmission electron microscope shows SARS-CoV-2, the virus that causes COVID-19, isolated from a patient in the U.S. HANDOUT/AFP/Getty Images

A new analysis by a team of Canadian and international researchers suggests that the novel coronavirus can be transmitted by infected individuals before symptoms develop – a possibility that could explain why the spread of the epidemic has proved so difficult to contain after it first appears in a new location.

If accurate, the findings pose a challenge to health officials who are assessing their strategies for dealing with the COVID-19 epidemic.

“It makes this very different from something like SARS,” said Caroline Colijn, a co-author on the study and an applied mathematician at Simon Fraser University in British Columbia who specializes in modelling infectious disease. “It means we would not be able to stop all the transmission events by focusing on cases who have already developed symptoms.”

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One reason that the SARS epidemic in 2003 was ultimately contained to about 8,100 cases worldwide is that those who were spreading the infection already knew they were sick. As of Tuesday, the World Health Organization reported that more than 113,000 people have contracted COVID-19.

The new analysis underscores the importance of social distancing and other strategies that seek to limit opportunities for contact with individuals who may be infected but not realize it – although Dr. Colijn acknowledged that such measures are among the most disruptive to daily routines.

EFFectiveness OF SOCIAL distancing during pandemics The authors of a study, published in Emerging Infectious Diseases, conducted a systematic review of the effectiveness of six social-dis- tancing measures during past flu pandemics. They concluded that early implementation delayed the peak in the number of infections, relieving the burden on health-care systems by spreading out the cases over a longer period of time. Without measures With measures Number of infections Delaying the peak of infections reduces the burden on health-care systems Capacity of health-care system Cases are spread out over longer period Time since first case identified JOHN SOPINSKI/THE GLOBE AND MAIL, SOURCE: fondazione gimbe (via m.w. fong et al, cdc, emerging infectious diseases) EFFectiveness OF SOCIAL distancing during pandemics The authors of a study, published in Emerging Infectious Diseases, conducted a systematic review of the effective ness of six social-distancing measures during past flu pandemics. They concluded that early implementation delayed the peak in the number of infections, relieving the burden on health-care systems by spreading out the cases over a longer period of time. Without measures With measures Delaying the peak of infections reduces the burden on health-care systems Number of infections Capacity of health-care system Cases are spread out over longer time period Time since first case identified JOHN SOPINSKI/THE GLOBE AND MAIL, SOURCE: fondazione gimbe (via m.w. fong et al, cdc, emerging infectious diseases) EFFectiveness OF SOCIAL distancing during pandemics The authors of a study, published in Emerging Infectious Diseases, conducted a systematic review of the effectiveness of six social-distancing measures during past flu pan- demics. They concluded that early implementation delayed the peak in the number of infec- tions, relieving the burden on health-care systems by spreading out the cases over a longer period of time. Without measures With measures Delaying the peak of infections reduces the burden on health-care systems Number of infections Capacity of healthcare system Cases are spread out over longer time period Time since first case identified JOHN SOPINSKI/THE GLOBE AND MAIL, SOURCE: fondazione gimbe (via m.w. fong et al, cdc, emerging infectious diseases)

“It can feel like ‘Oh, that’s too much’ before the problem, and then after the problem it can feel like ‘Oh, that wasn’t enough'” she said.

An important caveat is that the study, which is available online, has been submitted for publication but has not yet undergone peer review. However, the result is supported by at least one report published last month that documented the case of a five-member family group in China in which four of the family members appear to have been infected by another who showed no symptoms of COVID-19.

The analysis conducted by Dr. Colijn and her colleagues offers a broader and more statistical look at the question of asymptomatic transmission. It is based on data from 135 confirmed cases in Tianjin, China, and an additional 93 cases in Singapore. In both locations, the researchers drew on details in case reports related to contact history and the appearance of symptoms. These were used to mathematically determine two key numbers that are important for predicting the spread of an epidemic.

The first number is incubation period: the time between contracting the virus and the development of symptoms. In their analysis of the combined data, the researchers found that the average incubation period for COVID-19 falls between six and 10 days. (It can also last longer in some cases, which is why a 14-day quarantine period has become standard practice for those who may have been exposed to the coronavirus).

Their result is similar, although slightly longer than an average incubation period of five days, which was calculated by a team from Johns Hopkins University that published its result on Monday.

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Dr. Colijn and her colleagues then determined a second number, know as the serial interval – the average time it takes to communicate the virus from person to person in a chain of infection. This worked out to be about 4.5 days, with about a day and half of uncertainty on either side.

Although the two numbers are close, with uncertainty ranges that slightly overlap depending on location, a key takeaway from the study is that the serial interval for COVID-19 appears to be shorter than the incubation period. Such a result would virtually guarantee that the virus can sometimes spread between individuals before symptoms show up.

While such a situation would compound the challenge of containing the virus, it also comes with one small bit of good news, Dr. Colijn said. If the virus can spread asymptomatically it means that it is probably somewhat less contagious than current estimates suggest. Otherwise, asymptomatic transmission would have already produced a higher case count and more rapid spread of COVID-19 than has been seen so far.

Justin Lessler, an epidemiologist and co-author of the Johns Hopkins study, said that the figure that Dr. Colijn and her colleagues obtained for the serial interval of COVID-19 “seems a little short … but not completely implausible.”

He added that determining with certainty if people who show no symptoms can transmit the virus is among the most important details that still need to be understood about COVID-19.

This is not the first time researchers have raised the possibility of asymptomatic transmission. In late January, as the coronavirus epidemic was rapidly escalating in China, German researchers concluded that four cases of what is now known as COVID-19 that appeared in the Munich area had been caused by contact with a Chinese colleague who showed no signs of illness.

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That study proved to be a red herring. It later emerged that the individual who had triggered the mini-outbreak was already feeling unwell while en route to Germany.

Since then, the stealthy and sometimes rapid progress of the coronavirus beyond China’s borders has once again raised concerns about the possibility of asymptomatic transmission and created an urgent need to pin down the basic characteristics of the developing epidemic.