Did Thanksgiving Spread Covid-19? Here’s Why You Can’t Tell Yet.

Did Thanksgiving gatherings and travel accelerate the spread of the coronavirus in the United States, as many experts had feared? When will we be able to tell?

Those questions do not yet have clear-cut answers, but one thing is certain: The daily figures over the next few days won’t prove anything. It takes time for infections to take hold, time for tests to detect them, and time for results to be reported.

In interviews, four experts agreed that there probably will be an upward bump in cases linked to the holiday, similar to the rises that were seen after Memorial Day and July 4. But they had different estimates for when it would emerge.

Part of the uncertainty stems from the virus itself. Its incubation period — the time after a person catches it but before symptoms appear — can range from two days to two weeks or longer, though five days seems to be typical.

People who suspect infection may then wait to be tested, and test results may take days to come back from a lab.

Dr. Thomas Russo, a professor of medicine at the University at Buffalo, said that taking those factors into account, he expects a holiday bump to become noticeable at the end of this week, and continue through much or all of next week.

“Most people were being good and had celebrations just within their own households,” he said. “But that significant minority that did not is driving this surge, and will be our undoing.”

He noted that around 97 percent of infected people who develop symptoms do so within 12 days after exposure — and will already have been contagious for a day or two.

Others will feel no symptoms at all, an added complication that has dogged public health officials since the pandemic began. Those people may not quarantine or be tested — but they can infect others, lengthening the chain of transmission.

That’s what happened after July 4, said Megan L. Ranney, an emergency physician and public health researcher at Brown University. She said a jump in positive tests began between two and four weeks after the holiday, suggesting that many were pass-along infections.

The American Automobile Association forecast that about 50 million people would travel for Thanksgiving. Even if only 1 percent caught the virus, Dr. Ranney said, “that’s an extra 500,000 infections in one day,” and they could infect untold thousands more before showing up in the statistics. “We are looking at an exponential effect,” she said, one that would only truly be seen around Christmas and New Year’s Eve. “It will be a double whammy.”

Lewis S. Nelson, chair of the Department of Emergency Medicine at Rutgers New Jersey Medical School, said he was not certain that Thanksgiving travel and gatherings would create a widespread surge in new cases. The virus has been tricky, he said, and predicting the numbers can be extremely difficult.

“We keep saying we’ll see a bump, but most of these events don’t seem to really materialize into something really concerning,” Dr. Nelson said. “Sometimes what you expect to happen doesn’t happen.”

Even so, he said, “my gut tells me we should remain concerned and attentive.”

Dr. Nelson said his hospital in Newark, N.J., was overwhelmed in April, but is manageable now, even though the state is reporting many more new cases. He said there were currently about 30 coronavirus patients, compared to more than 200 in April, a drop he and his colleagues were still puzzling over.

“Just the whole fact that we are not experiencing April right now is inexplicable at this point,” he said. “People haven’t changed. A lot of us believe maybe the virus has.”

Officials were trying to make sure that any Thanksgiving exposure would be reflected in the data. On Monday, Mayor Bill de Blasio of New York City urged residents who had ignored official guidance and attended Thanksgiving gatherings to get tested.

In anticipation of a renewed demand, the city has opened 25 new testing locations in the last week. It will also now post online the wait times at its testing sites, which had seen growing lines as New Yorkers scrambled to get tested before their holiday plans.

The city’s seven-day average positive test rate was at 4.03 percent, Mr. de Blasio said, but he warned that the data may be skewed because fewer tests were conducted during Thanksgiving weekend.

Similar issues are clouding daily statistics across the country, as some states held off reporting for some or all of the holiday weekend and then caught up with big batches of new reports. Experts warned against drawing too many conclusions from the daily figures until states get back to a normal reporting rhythm.

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