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Will Thanksgiving cause an increase in COVID-19 cases? It did in Canada.

Lessons of Canada's Thanksgiving COVID surge
Coronavirus cases rose rapidly after Canada's Thanksgiving gatherings 07:44

Canadians celebrated their Thanksgiving last month, and if the day can teach anything to Americans, it's that gathering for the holidays can lead to an increase in COVID-19 cases. Canada saw an increase of positive coronavirus tests two weeks after its holiday — the typical incubation period for COVID-19 — and experts say this trend may be foreboding for the U.S.

On October 12, the day Canada celebrated Thanksgiving, the country had recorded about 185,300 total cases, according to data from Johns Hopkins University.

The number of total cases, which was already increasing, continued to climb; 4,000 new daily cases were recorded exactly two weeks later on October 26. On this day, the total number of cases that Canada recorded was about 222,900, per Johns Hopkins.

Dr. Laura Rosella, associate professor and epidemiologist at the Dalla Lana School of Public Health at the University of Toronto, said there is evidence that Thanksgiving gatherings are partly to blame for the increase.

"We were seeing an increase of cases leading up to Thanksgiving," Rosella told CBS News. "Cases were indeed increasing already, but we definitely saw an increase in the rate of transmission after Thanksgiving. And we know that Thanksgiving is important for a couple of reasons. One is through contact tracing data."

Rosella said several local public health authorities, which were tracking COVID-19 transmission through contact tracing, reported significant transmission between households during Thanksgiving. "For example, there was one local health unit that reported about 12 people being infected because of a Thanksgiving gathering," she said.

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There are several examples of this happening, Rosella continued. "We've seen quite a big acceleration in the two weeks following Thanksgiving and it's not the only reason the cases are increasing, it's not the only setting in which transmission is occurring, but definitely when people gathered indoors it did transmit COVID," Rosella said.

She added that the increase in cases occurred despite the fact that testing rates in Canada did not increase. Rosella also said many people avoided gathering in large groups, meaning the numbers could have been worse had more people disregarded guidelines. 

Canada has seen 305,449 total cases and 11,075 deaths, according to Johns Hopkins, while the U.S. has significantly more cases — with a total of 11,218,305 cases and 247,263 deaths. Rosella said when community transmission is high, like in the U.S., the probability of a in-person gathering that results in a significant outbreak is also higher.

So, Americans should heed this warning from Canada: Thanksgiving gatherings could lead to significant spread of the virus.

"We know that the highest risk situation to be in is when you're indoors and within close quarters," she said. "And if that is a situation people are contemplating, they're definitely putting themselves and their family at risk."

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While some people, like the elderly and those with preexisting conditions, might be more at risk for COVID-19, the virus is "unpredictable," Rosella said. She added that even if someone thinks they are gathering within safety guidelines, they are taking a chance and can still transmit the virus.

"There's been a few stories from the U.S. about weddings and big 'superspreader' events, and it might be the case that everybody in that circle or household ends up recovering, but they might come in contact with someone who doesn't," Rosella said. "So the knock off effects of that gathering could actually be significant and pretty detrimental to certain groups of the population."

"As we approach U.S. Thanksgiving, if people can keep in mind not just themselves or their family members or even the people they're gathering with, but maybe the vulnerable people that would be impact if a transmission were to happen in that household, I think that would be very wise and would mitigate a lot of heartbreak," Rosella said.

Thanksgiving warning to Americans

Epidemiologist Jennifer Nuzzo said Canada's numbers certainly serve as a warning to Americans. "Even without the data from Canada, this is a warning. We have reason to worry about Thanksgiving in the U.S. just on the principals of it," Nuzzo told CBS News.

"We are now seeing record case numbers being reported by nearly every state in the U.S.," said Nuzzo, an associate professor at the Johns Hopkins Bloomberg School of Public Health. "And the prospect of people at this point potentially traveling to spend time with people who have recently been in contact, to probably do their socializing indoors ... they're probably going to be sharing a meal and they probably won't be wearing a mask while sharing that meal. All those factors really stack up for it to be a really risky event for transmission to occur."

U.S. health experts have been warning against large indoor gatherings with people outside of someone's household. The Centers for Disease Control and Prevention released guidelines on how to make holiday celebrations safer if you see people outside your household.

Still, the CDC said the safest way to celebrate the holidays is by only seeing people you already live with.

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Should I get a COVID test before Thanksgiving?

For those planning to get a COVID-19 test before gatherings, Dr. Adnan Munkarah, chief clinical officer at Henry Ford Health system in Detroit, said people who receive a negative test might still not be in the clear.

"A COVID-19 test is only a snapshot in time and is not a reliable way to predict if it would be safe for people to gather together," Munkarah said in an email to CBS News. "It takes several days after exposure for a person to test positive for COVID-19."

He said the test may have been administered before the virus was detectable or a person might be exposed between the time they take a COVID-19 test and the time of the event. So, following health and safety guidelines and avoiding indoor gatherings is the best bet to prevent transmission.

Nuzzo said positive coronavirus cases are growing "closer to exponentially each week," so if the U.S. doesn't implement any new measures, "we could continue to see a compounding of new cases each week." She said it is up to the governors to expand restrictions if their states are seeing a surge in cases.

"It's almost impossible to fathom we are where we are right now in terms of the cases numbers," she said. "At the rate at which case numbers are increasing each day and the speed at which we are accumulating new cases, I think we should be quite worried about what is going to happen in the weeks to come, particularly if people use next week and the holidays that follow to potentially increase their exposure."

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