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Infectious disease experts warn that we could soon be paying the price for last year’s relatively nonexistent flu season.

Last winter, epidemiologists worried that a COVID-19 winter wave could coincide with the annual influenza outbreak, leaving hospitals and health care workers overrun with patients suffering from two different infectious diseases. But wearing face masks, social distancing, avoiding indoor gatherings, and other practices put in place to curb contraction of COVID-19 largely served to keep the U.S. population safe from influenza as well, according to public health experts.

The Centers for Disease Control reported that just 0.2% of the roughly 820,000 respiratory specimens tested between September 2020 and May 2021 came back positive for an influenza virus. During the last three flu seasons prior to the pandemic, the positive rate hovered between 26.2% and 30.3%, according to the CDC. There were also the fewest number of influenza-associated hospitalizations in the 2020–21 season—flu season is typically considered to start in October and go through the spring—since that type of data began being recorded in 2005.

Unfortunately, that may make the general public even more susceptible to the flu this time around, according to experts, especially if people do not get their flu shots.

“Because there were fewer flu cases in the U.S. last year, we should be expecting a reduced population immunity due to the lack of flu virus activity,” said Dr. William Checkley, who works as both a pulmonary and critical care physician at the Johns Hopkins Hospital and associate professor of medicine at Johns Hopkins University. “It could result in an earlier and possibly more severe flu season.”

Of course, safety measures such as wearing masks, social distancing, and getting a flu shot could once again help minimize influenza outbreak, according to Checkley. But as communities loosen COVID restrictions across the country—and as students return to school, employees return to the office, and fans return to stadiums—people may become more susceptible to infection.

And even though new cases of COVID-19 are declining, a flu outbreak could further burden health care workers already exhausted from 18 months of fighting the pandemic, according to Dr. Gregg Sylvester, chief medical officer of global influenza vaccine producer Seqirus.

“When I talk to my colleagues that are still in the trenches, they’re tired. They’re worn out,” Sylvester said in an interview. “They’ve been working far more than they ever had expected, and the last thing that we want to do, while they’re working hard to get the pandemic to go away, is to allow influenza to come in and create some problems. Whether it will ever be a ‘twindemic,’ I don’t know, but we certainly don’t want hospitals filled up with influenza cases.”

Scientists at the University of Pittsburgh Graduate School of Public Health recently released two analyses that predict a more severe flu season this year than in seasons prior to the pandemic.

Not including last year, flu seasons typically result in roughly 300,000 to 500,000 hospitalizations, according to the CDC. The Pittsburgh Public Health analyses found that the coming influenza season will likely result in 100,000 to 400,000 more hospitalizations than usual. However, both studies also suggest that “increased flu-related hospitalizations and deaths can be mitigated if vaccination rates are between 20% and 50% higher than those in recent flu seasons,” according to a press release.

“As COVID-19 containment measures—such as masking, distancing, and school closures—are relaxed around the world, we’re seeing a fierce resurgence of other respiratory viruses, which does not bode well for the coming flu season,” Dr. Mark Roberts, who is the director of the Public Health Dynamics Laboratory at the University of Pittsburgh and senior author on both studies, said in the release. “In a worst-case situation with a highly transmissible flu strain dominating and low influenza vaccination uptake, our predictive models indicate the potential for up to nearly half a million more flu hospitalizations this winter, compared to a normal flu season. Vaccinating as many people against flu as possible will be key to avoiding this scenario.”

Mutating strains

While health experts agree that we’re unlikely to experience another flu season as mild as last year’s, most acknowledge that the severity of any given flu season is difficult to predict with certainty, as the strains of the influenza virus mutate from year to year.

Because of the mutating nature of the influenza virus, some argue that reduced population immunity due to a lack of exposure to last year’s influenza virus may not affect the severity of this flu season. “It’s hard to say when a virus mutates that quickly really how much natural immunity was going to protect you to start with,” said Dr. Stu Coffman, an emergency physician and senior vice president at the national medical group Envision Healthcare. “So, I’m not sure that I would lay a lot of money on that. I would say that for sure the thing that you can do to protect yourself is to get vaccinated.”

Others worry that this year’s flu shot may not be as effective as in prior years. A lack of influenza cases earlier this year made it difficult to identify which strains need to be fought, according to Dr. Gregg Miller, chief medical officer of the health care staffing firm Vituity.

“Since there wasn’t a large amount of influenza circulating earlier this year when vaccines were being designed and manufactured, it was harder to determine what strains should be used in manufacturing the vaccine,” Miller said. “We’re not sure how effective the vaccine will be against whatever influenza strain becomes dominant.”

The CDC says that flu vaccination typically reduces the risk of flu illness by 40% to 60%, though vaccination effectiveness can vary.

Pediatric concerns

Even so, pediatricians are urging parents to get their children vaccinated. Last year there was just one influenza-related pediatric death. In the three years prior, there were roughly 150 to 200 deaths.

With children returning to schools and classrooms—some for the first time since the onset of the pandemic—there are more opportunities for the flu to spread among children, say experts.

“The vast, vast majority of children that die from influenza in childhood are not vaccinated,” Sylvester said. “And that’s the real sad story there, so my recommendation as a pediatrician—more importantly, the CDC’s recommendation—is that everyone 6 months of age and older get vaccinated for influenza every year.”

A 2017 CDC study found that flu vaccination reduced risk of flu-associated death by 51% among children with underlying high-risk medical conditions and by 65% among healthy children. The study also found that vaccinated children made up only roughly 25% of pediatric influenza-related deaths.

In addition to its recommendation that those 6 months of age and older get their flu shots, the CDC also noted that for children who are eligible, the COVID-19 and influenza vaccines can be co-administered.