En español | If you’re thinking about skipping your flu shot this fall because the flu was almost nonexistent last year, think again. Fewer cases from last season, experts say, may actually make the coming influenza season worse than normal. “There is thought we may be facing a more severe flu season because we didn’t have as much flu last year,” says Clare Rock, an infectious diseases physician and hospital epidemiologist at Johns Hopkins University School of Medicine. “That means some of the natural immunity that would be in the population is not going to be in the population now. We may have more vulnerable people.”
Rock and other experts say it’s too early to know for sure what the 2021-2022 flu season will look like, because other factors will also come into play: Will Americans continue to take COVID-19 precautions such as staying home, social distancing and wearing masks? How many will get the flu vaccine, and will it be a good match with circulating viruses? And what kind of impact will kids being back in school have on flu transmission?
“There’s a lot of uncertainty about this year’s flu season,” says Lauren Ancel Meyers, an epidemiologist at the University of Texas at Austin. “I wouldn’t bet one way or another at this point until we see the data start to come in.”
The uncertainty has health experts ramping up their plea for Americans to get their flu shots, especially since hospital beds are already filling with a surge of COVID-19 patients infected by the more contagious delta variant.
Virtually no flu season last year
Last year, for the first time in recent history, the U.S. Centers for Disease Control and Prevention (CDC) reported almost no cases of flu. Of 818,939 specimens tested for influenza from late September through late May, only 0.2 percent came back positive, compared to 26 to 30 percent during a normal season, CDC data show.
The CDC says the decline was most likely due to COVID-19 precautions such as wearing face masks, social distancing and people staying home from work and school. In addition, about 52 percent of American adults — a record number — got the flu shot last year.
With coronavirus cases spiking, the CDC is again recommending wearing face masks indoors, social distancing and avoiding crowds, even for those fully vaccinated against COVID-19. If Americans follow those recommendations and continue to take COVID-19 precautions into the fall and winter, that should also help suppress flu transmission, experts say.
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Schoolkids typically drive transmission
However, there are a few important differences this year compared to last year, experts say. First, more Americans are traveling, including to international locations where influenza may be circulating. Also, many restaurants, entertainment venues, colleges and other businesses are open, says William Schaffner, medical director of the National Foundation for Infectious Diseases and a professor of infectious diseases at Vanderbilt University Medical Center.
Perhaps most important, kids are returning to school in larger numbers and with fewer precautions than last year. “Kids often are that first wave of transmission in a community,” Meyers says. “They go to school … they spread the virus to one another, they bring it home and spread it to their families, and then it gets out into the community.”
Studies also show that children shed more flu virus for a longer period than adults, Schaffner says.
Compared to last year, “it looks much more like a conventional fall, with an array of activities that will give the flu virus opportunities to spread,” Schaffner says.
Predictions from the Southern Hemisphere?
Normally, scientists can look to the Southern Hemisphere, which has most of its flu cases during our summer, to get a good idea of how our flu season will unfold. But many countries south of the equator still have strict COVID mitigation measures in place, and that makes it more difficult to predict what will happen here, Rock says.
Australia, for example, has extremely tight border control and has implemented strict lockdowns to control COVID-19 outbreaks. Likewise, in many Southeast Asian countries, mask-wearing is more accepted and ubiquitous than in the United States.
Those measures may keep flu cases low in those countries but don’t necessarily foretell a similar trend for the U.S., especially if Americans as a whole take fewer precautions. “A really important variable is that behavioral variable,” Meyers says, “and we don’t have a crystal ball for the policies that will be put in place and the behaviors people will adopt over the next few months.”
Will this year’s flu vaccine be effective?
Another factor that will affect the severity of the flu season is how well the 2021-2022 vaccine matches circulating strains. So far, there appears to be less genetic diversity among circulating flu viruses than normal, which is a good sign, Meyers says. “If there hasn’t been that much evolution [of the virus], if there’s not that much genetic diversity, it may mean that our current vaccines are going to be a better match for the circulating strains and therefore offer better protection,” she says.
All the approved flu vaccines this year will be quadrivalent, which means they offer protection against four strains of flu, according to the CDC. Two of the strains are new compared to last year’s vaccine.
Adults age 65 and older should get the high-dose flu vaccine or the adjuvanted flu vaccine, the CDC says; both have been shown to evoke a stronger immune response in older adults.
Health care providers also hope to continue to boost the number of Americans who get the flu vaccine and hit another record this year. To make it easier, if you are eligible to get a COVID-19 vaccine booster in September or October, you can get the flu vaccine at the same time, Rock says.
“The more people who are vaccinated, the less flu circulates and the more protection there is for everybody, including the elderly,” Rock says.
Why you may want to get your flu vaccine early
If unusually low flu levels from last year indeed create a larger pool of susceptible people, with less natural immunity, experts say there could be a rise in cases earlier in the flu season. Other respiratory illnesses that normally peak in winter, including one that can be dangerous for older adults called respiratory syncytial virus (RSV), have already seen an early surge after historic lows last year.
That’s leading some experts to recommend getting your flu vaccine on the early side, say in September instead of October. The CDC recommends that all Americans age 6 months and older aim to get a flu vaccine by the end of October. (But it’s better to get it late than not at all.)
“We don’t know exactly if, when, how large or how severe this flu season is going to be, but with RSV, we saw an unusually early surge in transmission, and the same might happen with flu,” Meyers says. “Students are just starting to go back onto campuses. In the worst-case scenario, that could fuel not only large, but early waves of flu transmission. Because of that uncertainty, people should absolutely vaccinate, and vaccinate early.”
Michelle Crouch is a contributing writer who has covered health and personal finance for some of the nation’s top consumer publications. Her work has appeared in Reader’s Digest, Real Simple, Prevention, The Washington Post and The New York Times.