The flu all but disappeared amid the Covid-19 pandemic, thanks in large part to social distancing measures aimed at reducing the coronavirus's spread. But experts say there could be a downside: Scientists may struggle to predict what flu strains will dominate the next flu season—making it challenging to create effective vaccines.

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A 'strikingly different' flu season

According to Rachel Baker, who studies public health and infectious disease at Princeton University's High Meadows Environmental Institute, this year's flu season was "strikingly different," with nearly no flu in the Northern Hemisphere during the winter of 2020-21. Nor was there much of a flu season in the Southern Hemisphere, according to Ian Barr, deputy director of the World Health Organization's (WHO's) Centre for Reference and Research on Influenza at the Peter Doherty Institute for Infection and Immunity.

According to CDC, the hospitalization rate for the 2020-21 flu season in America was just 0.7 per 100,000 people—the lowest since the agency started tracking flu data in 2005. In fact, over the course of the last flu season, there was just one pediatric flu death—compared to 196 in the 2019-20 flu season.

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Part of the reason for that sudden drop-off could simply be a lack of testing for influenza, as health officials had to focus their resources on addressing Covid-19, John McCauley, director of the Worldwide Influenza Centre at the Francis Crick Institute, said.

But a major reason for that decline is the social distancing implemented to stave off Covid-19. Baker and her colleagues estimated that these measures led to a 20% drop in respiratory syncytial virus (RSV) transmission throughout the United States—and it likely was similarly effective at stemming influenza spread.

"This is one of the clear signals that these interventions work," Baker said. "They are really efficient at stopping the spread of these viruses."

But that silver lining may come at a cost

While the historically low levels of flu spared the country from what experts called a "twindemic"—a strong flu season hitting amid an ongoing Covid-19 outbreak—the pandemic's silver lining may come at a cost. According to Politico, the lack of a 2020-21 flu season means that scientists have little data to create effective vaccines in anticipation of the 2021-22 flu season.

Traditionally, to predict which flu strains will dominate the upcoming flu season, WHO convenes experts twice a year, once each for the Northern and Southern hemispheres, who examine data on flu strain spread collected by labs located around the world, including CDC.

During the meeting for the Northern Hemisphere, experts use data on what strains are affecting the Southern Hemisphere to predict which strains will hit the northern half of the globe a few months later. Then, an FDA advisory committee reviews those recommendations to decide how it will license flu vaccines for the coming year, Politico reports.

This year, the committee struggled to find any appropriate precedent. "What we asked … during that meeting was, 'Has there ever been a moment like this one?'" Paul Offit, a vaccine expert at the University of Pennsylvania and committee member, said. He added, "[T]his really is unprecedented."

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The closest flu season the United States has had to the 2020-21 season was the 2011-12 season, according to CDC, which set records at the time of the lowest and shortest flu peak. However, the numbers from the 2020-21 flu season are two-thirds lower than the rates of the 2011-12 season, CDC reports.

The good news is that the 2011-12 flu season didn't precede a bad flu season in 2012-13, nor did vaccine efficacy decline during the 2012-13 season—but it's still too early to anticipate how the 2021-22 season will play out.

For his part, Offit believes the flu levels this year still provided enough information for the committee to determine which strains the flu vaccine should protect against. "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," he said.

But in the absence of data, other experts are proposing staying the course from the previous flu season. For instance, Barr and McCauley supported keeping three of the four strains in the 2021-22 vaccine the same as they were in 2019-20's version for the Southern Hemisphere.

According to McCauley, that approach essentially banks on the idea that fewer infections in the 2020-21 season means the influenza virus didn't have many chances to mutate, and therefore the 2021-22 season will likely have the same dominate strains.

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How bad will upcoming flu seasons be?

Some experts have voiced concerns about the upcoming flu season in the context of a "pandemic-weary country," Politico reports—especially if a highly effective flu vaccine can't be developed. For instance, Lawrence Gostin, a global health law professor at Georgetown University, said as country starts to fully emerge from the pandemic, "we may have a combination of low public health measures at the population level with a low effectiveness vaccine," potentially resulting in "a raging flu season next year."

On the other hand, it's possible that the population may become more comfortable using social distancing measures every flu season to stem the spread of influenza. But some experts are skeptical the lessons of 2020 will stick.

"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 … comfortable having hundreds of thousands of cases of hospitalizations for flu and tens of thousands [of] 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."

Gostin agreed, saying that pandemic fatigue could lead people to stop wearing masks and stop social distancing during flu season.

"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" (Hilton, Politico, 3/25; Owens, Medscape, 3/3).