Even after quarantining for 14 days a small but potentially dangerous number of people will still carry the virus that causes COVID-19 and risk infecting others, a University of Victoria infectious disease modelling expert has found.

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“We think 22 days would be safer, but 22 days is a long period of time,” said Junling Ma, an associate professor in the department of mathematics and statistics who has modelled the spread and optimal control strategies for diseases like influenza, HIV, Ebola and cholera.

Ma and his colleagues looked at 646 COVID-19 case studies from China, including some from Hong Kong. The sample did not include cases from Hubei province, which has been the centre of that country’s outbreak.

In about 7.3 per cent of patients they found the incubation time from infection until a person developed symptoms was 14 days or longer.

For about five per cent of people the incubation was even longer — 17 days or more.

That may not matter somewhere like Vancouver Island where the number of cases are small, Ma said, but in other places would be significant.

“If we’re in a situation like New York, five per cent would be a big number.”

Some people who have returned from travel or who believe they may have been exposed to the virus will pass the required 14 days in quarantine, then go on to develop symptoms after the waiting period is over and they’ve returned to work or other activities.

“With a large base, it is very likely that these patients will kindle new infections,” Ma said. “It is especially dangerous because other people will think that they are not infected as they have been quarantined. The possibility of asymptomatic transmissions only worsens the problem.”

Two weeks is the standard length of quarantine for COVID-19 around the world and is recommended by the World Health Organization. Previous research based on 181 confirmed cases in China excluding Hubei had found that the median incubation period for the virus is 5.1 days and that 99 percent of people developed symptoms within 14 days.

While Ma said his research, which he’s in the process of submitting to a peer-reviewed journal, indicates a longer quarantine period would be safer, he’s not recommending extending it at this point.

“Quarantine period is always a trade off between feasibility and safety,” he said. “Making the quarantine period too long may increase the chance that people leave quarantine early.”

Instead, Ma said, people should be encouraged to stay home longer if they can and made aware that even if they don’t show symptoms after 14 days of quarantine there’s still a small chance that they have the disease and could infect others.