In a series of tweets on Friday, Ashleigh Tuite, an epidemiologist at the University of Toronto, provided an illustration of how many COVID-19 infected travellers could arrive at the Canada-U.S. border in Ontario, depending on the rates of infection in the U.S. and how many travellers are crossing overall.
The model offers insight into how far the coronavirus case count in the U.S. would need to drop in order to safely reopen the border.
In the tweets, Tuite said currently the number of people crossing the Canada-U.S. border into Ontario is low, probably fewer than 1,000 to 5,000 a day.
On Saturday the U.S. Centers for Disease Control reported 74,818 new cases of COVID-19.
Tuite said if Ontario sees a thousand travellers a day at the U.S. border, approximately two of those people will be infected with COVID-19.
“And then if you increase that to 10,000 travellers every day we expect, on average, 17 people arriving that were infected with COVID,” Tuite told Global News.
She said about one-third of those cases would be symptomatic when they arrive at the border.
“So those ones you would potentially identify just by a border screening and asking people to identify that they had symptoms,” she said.
The other two-thirds of cases would either be in the incubation period or would be presymptomatic or asymptomatic.
“And for those cases, the biggest control that we have is quarantine,” Tuite explained.
Currently, anyone entering Canada must comply with a mandatory 14-day quarantine period.
Those who arrive in the country who are unable to prove they have a viable isolation plan will be taken by officials from the Public Health Agency of Canada to a facility to do so.
Tuite said if people are adhering to the rules, then the infected travellers “shouldn’t be an issue.”
“The challenge is that if they’re not in compliance with those quarantine measures, then any of those cases could potentially spark a small outbreak,” she explained.
In the series of tweets, Tuite wrote that once the case burden in the U.S. declines to between 1,000 to 10,000 infections per day, Canada can ease border restrictions while keeping the imported case numbers low.
But, she said it’s “really hard to predict” when that could be.
Tuite said in April and May it seemed as though things might be under control in the U.S., but that the country is seeing a spike in new cases again.
“So I don’t think you can really put a timeline on it,” she said.
For now, Tuite said she thinks it is safest that the border remains closed to non-essential travellers.
“And even with good adherence to quarantine and screening, you only need one or two of those cases to not comply, to potentially be associated with a super spreader event and really set off a much larger outbreak,” she continued.
Should Canadians be travelling?
In a previous interview with Global News, Colin Furness, an infection control epidemiologist and associate professor at the University of Toronto, said travel needs to be kept to a minimum as much as possible with no exceptions for any kind of tourism.
He said beyond just keeping the border closed to Americans, Canadians should not be coming and going as they please.
“We shouldn’t be having that,” he said. “It’s not just about keeping Americans out if we want to make things better,” he said.
“Canadians should not be leaving the country for tourism or business travel.”
U.S. cases spike
The United States remained the epicentre of the virus on Sunday.
According to a tally from Johns Hopkins University, by 12 p.m. ET Sunday, there were more than 4.1 million confirmed cases of COVID-19 in the country.