How Covid-19 Precautions Have Kept Flu Cases Low

By this time last year, the U.S. had recorded over 65,000 cases of influenza

A person stands by a sign advertising flu shots at CVS as the city continues the re-opening efforts following restrictions imposed to slow the spread of coronavirus on December 01, 2020 in New York City.
Mask wearing and other behaviors developed during the Covid-19 pandemic are largely keeping flu cases low this year. Photo by Noam Galai/Getty Images

The United States has recorded just over 1,000 cases of the flu since September 2020, which is unusually low, Sara Kiley Watson reports for Popular Science.

During the same time period last winter, the country recorded over 65,000 cases of the flu. But in the last year, the Covid-19 pandemic has changed a lot about the way that people act to prevent the spread of disease. Precautions like wearing masks, taking extra care when washing hands, and keeping activities outdoors and at a distance from others have all been implemented to slow the spread of Covid-19. The same measures may have stymied the spread of influenza and other seasonal viruses.

“I’d be very surprised if we have a typical flu season now,” says virologist John McCauley, the director of the Francis Crick Institute’s Worldwide Influenza Centre, to Science magazine’s Kelly Servick. “To see nothing so far, it’s difficult to see how it’s going to come up in large numbers in January.”

Countries across the Northern Hemisphere are seeing a quiet flu season. In England, flu cases are about one-twentieth of the usual cases at this time of year, Linda Geddes reports for the Guardian. The Southern Hemisphere saw a similar phenomenon between June and August, when influenza cases usually peak, the Centers for Disease Control and Prevention reported in September.

Behavioral shifts seem to be a main driver of the flu’s yearly spread. Normally as the weather turns cold, students return to school and people move their activities indoors.

“Since that isn’t happening, there’s definitely a trickle-down effect to other respiratory viruses circulating at this time,” says Johns Hopkins epidemiologist Elizabeth Lee to Popular Science.

Flu vaccination rates are also higher than usual this year. In 2019, about 42 percent of adults in the U.S. received the flu vaccine; this year, 53 percent have. That would also tamp down the spread of influenza, although it doesn’t account for the entire drop in flu cases this year, McCauley tells Science magazine.

The World Health Organization says that influenza is currently at “interseasonal levels,” according to Science, which means that the Northern Hemisphere is currently seeing about the same amount of flu as during summer months.

“This is real and reflects two things: overwhelmingly the main thing is that social distancing and lockdown measures dramatically reduce the transmission of cold, influenza and other respiratory viruses,” says University of Southampton primary care researcher Paul Little to the Guardian. In England, he adds, “There may be a smaller secondary effect in that people may be contacting their GP [general practitioner] less with ‘normal’ cold and coughs – but that cannot possibly explain the huge differences observed.”

In some cases, infections with one virus can cause a person’s immune system to block an infection by another virus, which makes it look like the two diseases take turns causing disease in a community. This can't be ruled out with Covid-19 and the flu, Lee tells Popular Science, but more research is necessary to show such a connection.

Low influenza cases might impact future flu seasons. For instance, an unusually high number of people might be susceptible to the flu viruses that appear next winter, although people could combat that by getting vaccinated again, per Science. There are also fewer samples of flu virus for researchers to study in order to build next year’s vaccine. But researchers have identified a strain of the flu virus that has caused a local outbreak in Cambodia, Bangladesh and India.

“That’s the strain I suspect would be the one we’d pick [as a component of] the vaccine if we had to pick it today,” says University of Washington virologist Trevor Bedford to Science magazine. A group of scientists with the World Health Organization will meet in February to plan next season’s flu vaccine.

But flu outbreaks are difficult to predict, and a lot depends on the Covid-19 pandemic. Bedford adds to Science that if not enough people become vaccinated against the coronavirus, then there might be another resurgence at the end of 2021 that would spark more lockdowns that have prevented flu transmission. And even if that worst-case scenario doesn’t come to pass, it remains to be seen what disease-preventing hygiene measures will stick around.

“I am sure that flu will come back with a vengeance at some stage in the future,” said Griffith University epidemiologist Robert Ware to Nicola Jones at Nature News in December, “but it might take a few years.”