Concerns over more contagious forms of the COVID-19 virus are causing some to wonder if even a very brief interaction can cause the illness to spread from one person to another.

Health Canada's guidelines for contact tracing -- the method of identifying potential spread from an infected case -- considers someone a close contact if they've been within two metres of a positive individual for 15 minutes or more.

But while that timeframe helps contact tracers do their job, it should not necessarily guide our behaviour, infectious disease experts say.

That's because it's hard pin down what a "safe" length of time for an encounter is, since factors like distancing, masking and ventilation will play into any scenario. Local health regions that have experienced outbreaks with the new variants are trying to determine if other factors contributed to a quicker spread.

While new variants of concern first identified in the U.K., South Africa and Brazil can transmit more easily than other strains, experts say brief interactions with an infected person -- whether they have a new variant or not -- can be risky under the right circumstances.

"A high-risk exposure is a high-risk exposure," said Dr. Zain Chagla, an infectious disease specialist with McMaster.

"I don't think setting some time expectation is necessary to say (transmission) can happen under 15 minutes."

The 15-minute threshold for contact tracing has remained the same throughout the pandemic and is in line with countries like the U.S.

But it is only a guideline, and some health units in Canada have already tweaked their benchmarks to reflect a more cautious approach.

Dr. Karim Kurji, the medical officer of York Region north of Toronto, says his health unit has used a 10-minute threshold for contact tracers, starting long before the new variants emerged.

"It's not just the timeframe we look at, however," he said. "(We see) whether one or both parties were wearing masks, whether they were talking in low tones or loud voices. There are a number of factors case investigators have to take into account."

York Region's total number of variant cases reached 39 this week, all of them the variety first found in Britain. The variant has also been detected in B.C., Alberta and Quebec, and was linked to a devastating outbreak at a long-term care home in the Simcoe Muskoka health unit that has led to 66 resident deaths.

Kurji said some of the people in York Region who got the variant seemed to have been "quite careful ... and acquired it nonetheless," which could mean brief interactions, like short shopping trips, were to blame.

Kurji said analysis of those 39 cases suggest the "attack rate" -- how many people one positive case goes on to infect -- could be 70 per cent higher than what his health unit has seen with the current dominant strain. He noted, however, that 39 cases is a small sample size.

The incubation period -- the time between infection and potential symptom-onset -- has been as low as 18 hours to two days among some York cases, Kurji said. The normal timeframe is five to seven days.

"So if these individuals were out in the community exposing others ... you can see how exponential the growth could be," he added.

Chagla says people infected by new variants may have a higher viral load, which could lead to easier transmission and could explain why some of the York cases may have caught the virus in lower-risk environments.

"It's still the same events that are leading to transmission, it's just that there seems to be less of a buffer, according to a few cases here and there," Chagla said. "So that's why I think public health units have been surprised."

Dr. Sumon Chakrabarti, an infectious disease expert in Mississauga, Ont., says while an accelerated rate of infection would be worrisome, the presence of the variants only highlights the importance of public health measures aimed at slowing COVID's spread.

Chakrabarti, who's been involved in contact tracing for the last couple months, says people sometimes think they caught the virus in perceived high-risk settings like grocery stores, but may have been infected elsewhere.

Still, he added, a brief interaction could "possibly" spread the variant.

"All this fervour happening with the variants, yes, there's evidence they're more transmissible, but (transmission) tends to still happen in areas you'd expect -- crowded workplaces, households," Chakrabarti said.

"I don't think you have to worry about being in a grocery store for a short period of time, but it does reinforce the importance of mask-wearing indoors and all those things we talk about."

Chagla says the relatively low prevalence of the variants in Canada will make it easier to pinpoint where spread is happening.

If someone thinks they were infected in a store and another shopper tests positive for the variant, that epidemiologic link will be easy to see, he says, as long as we catch both cases.

Still, experts say, adhering to current public health measures will be the best defence against any form of the virus.

And using a timeframe to determine risky interactions won't be helpful.

"In reality, five minutes in an elevator with poor ventilation could lead to transmission," Chagla said. "So there's nothing magical about Minute 15."

This report by The Canadian Press was first published Feb. 4, 2021.