The flu disappeared during the pandemic. What does the return look like?

Note: Figures reflect weekly totals of positive influenza studies from public and clinical laboratories.·Source: Centers for Disease Control and Prevention

There are fewer flu cases recorded in the United States this flu season than ever before. About 2,000 cases have been recorded since the end of September, according to data from the Centers for Disease Control and Prevention. In recent years, the average number of cases in the same period was about 206,000.

When measures were taken in March 2020 to stop the spread of the coronavirus across the country, flu quickly disappeared and it has still not returned. The last flu season, which would normally last until next month, essentially never happened.

After fears that a “twin chemistry” might hit the country, the absence of the flu was a much-needed reprieve that eased the burden of an overwhelmed health care system. But the lack of exposure to the flu can also make the population more susceptible to the virus when it returns – and experts say the return is certain.

“We don’t know when it will come back in the United States, but we know it will come back,” said Sonja Olsen, an epidemiologist at the CDC.

Experts are less sure about what will happen if the flu returns. In the coming months – as millions of people return to public transportation, restaurants, schools and offices – flu outbreaks could be more widespread than usual, they say, or occur at unusual times of the year. But it’s also possible that the virus that comes back is less dangerous because it didn’t have a chance to evolve while on hiatus.

“We don’t really have a clue,” said Richard Webby, a virologist at St. Jude Children’s Research Hospital in Memphis. ‘We are in uncharted territory. We haven’t had flu season this low, I guess as long as we measure it. So what the possible implications are is a bit unclear. ”

Scientists don’t yet know which public health measures were most effective in eradicating the flu this season, but if behaviors like wearing a mask and washing hands regularly after the coronavirus pandemic is over, they can help keep the flu at bay in the United States. States.

Much also depends on the latest flu vaccines, their effectiveness, and the public’s willingness to get them. However, the recent drop in the number of cases has made it difficult for scientists to decide which flu strains to protect against in those vaccines. It is more difficult to predict which species will circulate later, they say, when so few are circulating now.

What happened to the flu?

When the reality of the coronavirus pandemic kicked in last year, the country was still in the throes of the normal flu season, which had peaked in February. Then schools closed, travel stopped and millions of people started working from home, and the number of new flu cases quickly fell to all-time lows, even as the coronavirus soared.

Influence vs. coronavirus

Flu cases even dropped as the coronavirus spread.

Note: Figures reflect weekly totals.·Sources: Centers for Disease Control and Prevention (Flu cases); New York Times database of reports from state and local health authorities (coronavirus cases)

And the decline is not due to a lack of testing. 1.3 million specimens have been tested for influenza since the end of September, more than the average of about a million in the same period in recent years.

The public’s history of flu exposure, scientists say, may partly explain why the flu virtually disappeared while the coronavirus continued to spread after safety measures were implemented.

“For something like Covid, where you have a fully susceptible population at the start of a pandemic, it takes a lot more work to slow the spread of the infection,” said Rachel Baker, an epidemiologist at Princeton University.

In other words – unlike the coronavirus – the population has some natural immunity to influenza from years of exposure to different strains of the virus. People are susceptible to new strains of flu every year, but less so than to completely unknown viruses.

The mere presence of the coronavirus may also have played a role in suppressing flu cases, said Dr. Webby, because there is often only one dominant respiratory virus in a population at a time. “One tends to keep the other out,” he said.

And influenza wasn’t the only virus that disappeared in the past year; there were also significant decreases in other respiratory diseases, including respiratory syncytial virus or RSV, the most common cause of pneumonia in infants.

What happens if the flu returns?

Influenza is a relatively common disease that can be fatal, especially in young children, seniors, and adults with chronic health conditions. The CDC estimates that the flu has killed 12,000 to 61,000 people a year since 2010.

If flu immunity wanes during the pandemic due to the lack of exposure to the latest strains of flu, more people than usual could be susceptible to the virus.

“Every year somewhere between 20 and 30 percent of the population gets their immunity a little boosted and boosted by exposure to the flu virus,” said Dr. Webby. “We won’t have that this year.”

“Decline in natural immunity is a concern,” said Dr. Olsen, “and lower immunity can lead to more infections and more serious illnesses.”

The result could lead to larger outbreaks of flu and off-season RSV, said Dr. Baker. In Florida, RSV would normally be on the decline at this time of the year, but it is currently experiencing a revival.

As offices and schools reopen in greater numbers in the fall, as many expect, scientists will keep a close eye.

“We are always concerned that influenza causes serious illness, especially in individuals at increased risk of complications,” said Dr. Olsen. “We know that school-age children are an important driver of flu virus transmission. However, because flu is difficult to predict, we cannot predict next season’s severity. ”

There is also a potential benefit to the absence of influenza: fewer cases usually lead to fewer mutations.

“Right now, because influenza is not circulating that much, it is possible that the virus may not have had much opportunity to evolve,” said Dr. Baker, “which means our vaccines can be more effective than usual.”

The strains opt for the flu vaccine

Making the flu vaccine was more difficult this year than in the past.

Every year, scientists evaluate the strains of flu circulating around the world and come together to decide which strains to protect against in that year’s vaccine. They look at the types that make people sick and use that information to predict which types of people are most likely to infect people when the flu season begins.

“We met at the end of February to make those recommendations,” said Dr. Webby, referring to the World Health Organization panel reviewing the flu vaccine. And it was tricky. The amount of data was orders of magnitude less than normal. ”

Dr. Olsen, the CDC epidemiologist, pointed out that the vaccine choices are based on more than just existing strains. Scientists are also taking into account other data, including projections of “the likelihood of emerging flu viruses becoming more common in the coming months.”

And, she said, the uncertainty surrounding the flu recurrence makes flu vaccination more important, not less.

There is another hard-to-predict factor that could play an important role when the flu returns: whether society will continue the behaviors learned during the pandemic that benefit public health. Is wearing a mask becoming the norm? Are employers giving their employees more physical space?

The last time Americans had a chance to make that behavior part of the culture, Dr. Baker noted, it wasn’t.

“The 1918 flu pandemic should have been something that gave us some sort of social learning,” said Dr. Baker, but the behavior didn’t change. “So what’s the journey you will take after the Covid-19 pandemic, along that axis?” she added. “Do you want to wear your mask, even if no one else is?”