Feb, 25, 2021 -- Do you know anyone who’s had the flu this year? Probably not.

The U.S. is seeing historically low levels of influenza this season, which started in September 2020.

This time last year, the national map of flu activity published by the CDC showed so many active cases that some states had burned right through red to a dark purple for “very high” activity. This year, the map is a calm green with hardly a blip on the public health radar. Public health labs across the U.S. reported a grand total of 3 cases of flu in the U.S. last week, out of nearly 16,000 samples tested. Clinical laboratories, which tested nearly 25,000 samples, found just 14 flu cases.

So far this season, labs reporting to the CDC had just 1,585 samples test positive for flu of any kind. Compare that to last year over the same period, when there were more than 183,000 positive samples. Those numbers are making infectious disease specialists do double takes.

“Nobody has seen a flu season this low, ever. And some of us have some gray hair,” says William Schaffner, MD, an infectious disease expert at Vanderbilt University in Nashville. Vanderbilt is part of a network of hospitals that are actively looking for flu cases among their patients. They can’t find any.

So far this year, only one child has died of the flu. Last year, that number was 195.

Flu numbers are way down in Canada, too.

“It's absolutely incredible to have a complete nonevent flu season,” says Isaac Bogoch, MD, an infectious disease specialist at the Toronto General Hospital Research Institute in Canada.

Bogoch is Jewish, so every year, he volunteers to work over Christmas at his hospital to give his colleagues a break.

“I didn't admit one person with influenza during the Christmas block at Toronto General Hospital this year. And of course that's just my individual experience. But if you look at the national numbers for Canada, and of course the national numbers for the United States, it looks like everyone else had a very similar experience as me. There's just remarkably little influenza,” he says.

The pattern in North America followed an extremely quiet flu season the Southern Hemisphere. The U.S. typically looks to countries like Australia for a glimpse at what might be coming to our shores. Doctors say they are beyond relieved that we didn’t see flu misery on top of the winter COVID-19 surge.

“The question is why, and the answer is pretty interesting,” Bogoch says.

First, he says, there are the precautions people have been taking for COVID, like masking, social distancing, and frequent hand-washing. Sure, not everybody is doing these things, but many are. “I don’t think we can ignore that,” he says.

Secondly, many people seem to have heeded public health advice to get a flu vaccine.

“At least in Canada, there was massive uptake of the flu vaccine this year,” Bogoch says.

Official numbers on vaccination rates for this year’s flu season aren’t yet available for the U.S., but so far, the CDC says nearly 194 million doses of flu vaccine have been distributed. That’s another record, topping last year’s distribution by about 20 million doses. Distribution isn’t the same as the number of shots going into arms, so it remains to be seen whether high vaccination coverage may have played an important role in tamping down the flu here.

Bogoch also points to less international travel as a reason the flu wasn’t a big deal.

“This is an infection that that follows mobility patterns as well,” he says, “and there's just remarkably less human mobility this year.”

Perhaps the biggest reason for the disappearance of the flu this year has to do with children.

“Children are the great distributors of the influenza virus in our society,” Schaffner says.

Kids shed flu virus for longer than adults do, and they shed it a day or two before they show any symptoms, says Jennifer Nayak, MD, a pediatric infectious disease specialist at the University of Rochester Medical School. She has studied the life cycle of flu infections in children.

“As we’ve learned the hard way, with COVID, shedding virus before you are symptomatic makes it really, really hard to contain an infection,” she says.

Young children also don’t “control their secretions,” as Nayak gently puts it. They sneeze and cough and slobber all over the adult caregivers they are still highly dependent on, making them very good at spreading the flu.

But interestingly, kids may not play an outsized role in the transmission of COVID.

So far, evidence from household and school contact tracing studies of COVID suggests that children, especially younger ones, are often not the first in a group to get COVID and may not be as contagious as adults, though studies are ongoing to try to sort that out. Kids may also be less likely to show symptoms and be tested, making them silent spreaders of the infection.

And many kids were kept home this winter for virtual classes and birthday parties.

“They weren't even playing together, because mothers were keeping them off the playground and not having play dates. And so, in fact, I think that's the major reason,” the flu is all but gone, Schaffner says.

Show Sources

CDC: Fluview.

Isaac Bogoch, MD, infectious disease specialist,  Toronto General Hospital Research Institute.

William Schaffner, MD, infectious disease expert, Vanderbilt University, Nashville.

Jennifer Nayak, MD, pediatric infectious disease specialist, University of Rochester Medical School.

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