Canada's borders may be closing, but it's too late to prevent some COVID-19 cases. Three Westjet flights with COVID-19  either started or ended at Toronto's Pearson Airport on March 11-12, confirms the airline. This news comes just hours after Prime Minister Justin Trudeau announced non-Canadian residents and foreign travellers will be stopped from entering the country.

A tweet from WestJet on Monday, March 16 revealed four flights had guests recently test positive for the virus at the centre of a pandemic.

Three of those either originated or ended up at YYZ. 

A March 11 flight from San Juan, Puerto Rico to Pearson is one of the three in question. The others, both on March 12, were flight WS1201 from New York to Toronto and flight WS665, this one heading from Toronto to Calgary.

On the March 11 flight, rows 2 to 6 are thought to have been potentially exposed to the virus.

For flight WS1201, that range is rows 3 to 9, while for WS665, rows 19-27 are advised of the threat.

In a statement on its blog, WestJet stressed that passengers in the affected rows of the flights in question are seen as "close contacts" and should self-isolate for 14 days and monitor their symptoms.*

Meanwhile, according to CP24, one guest on board another March 11 flight, this time from London, England to Pearson, has also tested positive.

At-risk flyers arriving in Toronto should contact Telehealth at 1-866-797-0000, or reach out to their primary care provider or local public health unit.

Elsewhere, the airline confirmed another flight, WS2 from London, Ontario to Calgary on March 3, also held a passenger who has since tested positive. 

Just earlier on Monday, March 16, Trudeau announced that the Canadian borders will be closed to non-residents trying to fly into the country.

That does not, however, affect travel to or from the U.S. and Mexico.

It was also confirmed on Monday only four airports in Canada will be accepting international flights for the foreseeable future.

Those are Toronto Pearson Airport, Montreal—Pierre Elliott Trudeau International Airport, Calgary International Airport, and Vancouver International Airport.

Meanwhile, Canada's Border Services Agency will be stepping up its efforts in response to the growing pandemic through their screening in airports.

Passengers with COVID-19 symptoms may now be banned from flights entering Canada, but airlines are still having to thoroughly review their recent passengers right now.

*This article has been updated.

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