The pandemic dramatically reduced flu cases. That could backfire.

Once doors start opening again and people venture out without taking a year’s worth of Covid-19 precautions, it’s possible there could be new strains of the flu circulating that scientists didn’t anticipate, said Cody Meissner, an infectious disease specialist and pediatrician at Tufts Children’s Hospital who also serves on the FDA vaccine advisory panel. Without a strong enough vaccine, the pandemic-weary country could experience a severe flu season just as it emerges from fighting the coronavirus.

“We may have a combination of low public health measures at the population level with a low effectiveness vaccine. And then so you might have a raging flu season next year,” said Lawrence Gostin, a global health law professor at Georgetown University.

Concerns about the knock-on effects of the shockingly quiet flu season came to the fore earlier this month at a meeting of the Food and Drug Administration’s independent vaccine advisory committee. While experts suggested there are lessons to be learned from last year’s flu season that could help prevent high flu rates in future years, they also struggled to project what next winter could bring.

“What we asked them during that meeting was, ‘Has there ever been a moment like this one?’ Where there was very little flu circulating, which one can then make a judgement about what then happened the following year, but this really is unprecedented,” said Paul Offit, a vaccine expert at the University of Pennsylvania and a member of the FDA vaccine advisory panel.

Influenza is already a tough virus to track, Gostin said, because it mutates more rapidly than other familiar viruses such as measles. The flu’s quick-change act creates multiple strains every year.

The process of predicting which flu strains will predominate in each flu season is a global effort. The World Health Organization gathers experts twice a year to forecast flu strains — once each for the Northern and Southern hemispheres — based on data collected by labs around the world. They include the Centers for Disease Control and Prevention, which is one of WHO’s six “collaborating centers” for flu research.

At the Northern Hemisphere meeting in February, experts review what strains are then circulating in the Southern Hemisphere as it approaches autumn — and use that information to interpret which strains might hit the northern part of the globe months later. In the United States, the FDA vaccine advisory committee reviews those recommendations and makes a final decision about the makeup of the flu vaccines the agency will license.

Kawsar Talaat, an assistant scientist at Johns Hopkins with an expertise in infectious disease, said that experts watch how the virus evolves over the course of the season and which strains are dominant towards the end of the season.

There are few precedents for the most recent U.S. flu season. The 2011-12 flu season set records for the lowest and shortest peak of flu, according to the CDC. The numbers for the last season are still just one-third the rate of the 2011-12 season, the agency reports.

That previous low year did not seem to lead to particularly bad flu season in 2012-13, Talaat said, adding that the vaccine effectiveness that year was in line with or better than most years. She said it is too early to know what the flu season next fall will look like or to predict what the likely effectiveness of the flu vaccine is.

“In 2011-2012, we had almost no flu season and so that happens from year to year. And then last year we had a three-pronged, three-peaks flu season which was also very unusual. So influenza biology is a fascinating topic,” Talaat said.

Offit said that the low levels of the virus this season still yielded enough circulation for the FDA advisory committee to pick strains for a vaccine and that he is not worried about the vaccine for next fall.

“The belief is that there was enough circulating virus to be able to pick what is likely to be the strains that are associated with next year’s flu outbreak,” Offit said.

Influenza was not the only winter respiratory virus to see low numbers this season. Rates of respiratory syncytial virus were also down, Offit said. The disappearance of winter respiratory viruses is causing health experts to wonder if the Covid-19 mitigation strategies could become a necessary tool to fight them off every year.

“I mean, could we reasonably in a winter month, wear masks just at least when we’re outside in large crowds?” Offit said. “Did we learn that or are we willing to, or are we comfortable having hundreds of 1000s of cases of hospitalizations for flu and 10s of 1000s deaths? I suspect the answer is B. We’re comfortable with that, we’re willing to have that even though we just learned, there’s a way to prevent it.”

People are ready for life to get back to normal, Gostin said, and Covid-19 pandemic fatigue could have people ditching masking and social distancing at just the right time for flu.

“Remember after the 1918 flu pandemic, most people don’t realize what happened when that was over. But what happened was the roaring ‘20s,” Gostin said. “People started congregating, mingling, hugging, kissing. All the things they missed. They crowded into theaters and stadiums and went back to church. That’s what’s likely to happen this fall and that makes the influenza virus very happy.”