News that Canada may soon welcome fully vaccinated U.S. travellers comes as COVID-19 cases rise in some states south of the border, but infectious disease experts say the risk posed by vaccinated visitors is low.

Prime Minister Justin Trudeau indicated Thursday that Canada could reopen the border to fully vaccinated Americans by mid-August, with immunized travellers from across the globe following suit by early September.

Dr. Sumon Chakrabarti, an infectious disease specialist in Mississauga, Ont., said it's the next logical step in reopening plans, and would mark a shift from efforts to eliminate risk to those that mitigate it. 

"The risk won't be zero ... (but) we have to start making these adjustments to move back to normal," he said. "We can't stay in suspended animation with our nearest neighbour."

The U.S. recorded more than 35,000 COVID-19 cases on Thursday, up from 12,000 daily cases a month ago. The country was averaging 250,000 cases in January before its vaccine rollout sped up. 

Fifty-five per cent of Americans were at least partially vaccinated as of Friday, with 48 per cent fully vaccinated. Canada's vaccine rate was nearing 70 per cent of the total population, including 47 per cent fully jabbed.

A fully vaccinated person can still catch COVID-19, though it happens rarely and illness is less severe when it does occur. While vaccinated individuals can still spread the virus, that risk is also "significantly reduced," Chakrabarti said.

A recent study from the United Kingdom compared spread among household contacts after vaccinated and unvaccinated family members got COVID-19. The study found at least one dose cut transmission to unvaccinated members by 40 to 50 per cent.

Another pre-print study from Israel, which has yet to be peer-reviewed, suggests vaccines are 88.5 per cent effective against transmission.

"So it's clearly not 100 per cent, but (vaccination) really does (reduce) the transmission chain," Chakrabarti said.

Nazeem Muhajarine, an epidemiology professor at the University of Saskatchewan, said that while spread from a fully vaccinated traveller would be rare, unvaccinated or partially-vaccinated Canadians are vulnerable, especially if visitors unknowingly bring in new variants of the virus. 

Muhajarine, who's also one of the leads for CoVaRR-Net, Canada's research network on variants of concern, said a traveller's country of origin should still be considered regardless of their vaccination status. Visitors from Peru, for example, where the new Lambda variant is entrenched, could pose more risk.

"The fact that they are fully vaccinated, that's an important piece of information," Muhajarine said. "But we also need to pay attention to the epidemiological scenario of where they're coming from."

Details on how screening might unfold with a border reopening weren't yet released as of Friday, and there was no mention of whether U.S. visitors would be required to show proof of vaccination. The White House has already ruled out the idea of a vaccine passport.

Specifics on whether Canada would keep the negative-test requirement for anyone entering the country was also missing.

Dr. Ilan Schwartz, an infectious disease researcher with the University of Alberta, expects the negative-test provision to remain in play as a way to drive risk from "extremely low to even lower."

"But we're already starting from a place where, in the absence of symptoms or the absence of exposure to a known case, the likelihood of travellers importing the virus into Canada is low," he said.

Schwartz said Canada will also have to determine which vaccines it will accept in its definition of a "fully vaccinated" visitor.

Parts of the world, including Russia and China, use vaccines that aren't authorized by the World Health Organization and have "little or dubious public data," Schwartz said. But being "overly elitist" in qualifying vaccines could pose ethical issues, he added.

Muhajarine said as more countries begin excluding unvaccinated travellers from entering their borders, that could incentivize more people to get their jabs, elevating vaccine uptake in Canada and elsewhere.

But while Chakrabarti said limiting travel to fully vaccinated people makes sense, any missing details in the rules could spur confusion, especially among families hoping to travel with unvaccinated children.

"What happens if you have five members of a family who are vaccinated and one who isn't?" Chakrabarti said. "There's a lot of unknowns here and a lot of unintended consequences that this could lead to."

This report by The Canadian Press was first published July 17, 2021.

Melissa Couto Zuber, The Canadian Press