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

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

There have been fewer cases of influenza in the United States this flu season than in any of them. About 2,000 cases have been reported since the end of September, according to the Centers for Disease Control and Prevention. In recent years, the average number of cases in the same period has been about 206,000.

As measures to stop the spread of coronavirus were implemented across the country in March 2020, the flu has quickly disappeared and has not yet returned. The last flu season, which would normally have lasted until next month, didn’t really happen.

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

“We don’t know when he will return to the United States, but we know he will return,” said Sonja Olsen, an epidemiologist at the CDC.

Experts are less sure what will happen when the flu returns. In the coming months – as millions of people return to public transport, restaurants, schools and offices – outbreaks of the flu could be more widespread than usual, they say, or could occur at unusual times of the year. But it is also possible that the returning virus is less dangerous, did not have the opportunity to evolve while on hiatus.

“We have little clue,” said Richard Webby, a virologist at St. John’s Children’s Research Hospital. Memphis County. “We are in an unexplored territory. We haven’t had such a low flu season, I think as long as we measure it. So what are the potential implications is a bit unclear. ”

Scientists do not yet know what public health measures have been most effective in eradicating the flu this season, but if behaviors such as wearing masks and frequent hand washing continue after the end of the coronavirus pandemic, they could help keep the flu at bay. distance to the United States.

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

What happened to the flu?

When the reality of the coronavirus pandemic set in last year, the country was still in the throes of the normal flu season, which peaked in February. Then schools closed, travel stopped, and millions began working from home, and the number of new flu cases dropped rapidly to historic lows, even as the coronavirus rose.

Influence vs. coronavirus

Influenza cases decreased even as the coronavirus spread.

Note: The figures reflect weekly totals.·Sources: Centers for Disease Control and Prevention (influenza cases); New York Times database with reports from state and local health agencies (coronavirus cases)

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

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

“For something like Covid, where you have a completely sensitive population at the onset 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 coronavirus – the population has a natural immunity to the flu, from years of exposure to various strains of the virus. People are susceptible to new strains of the flu every year, but less so than they are to completely unknown viruses.

The mere presence of coronavirus could also have played a role in suppressing flu cases, Dr. Webby said, 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 the flu was not the only virus that disappeared in the last year; there have also been substantial decreases in other respiratory conditions, including respiratory syncytial virus or RSV, which is the most common cause of pneumonia in infants.

What will happen when the flu returns?

Influenza is a relatively common disease that has the potential to become fatal, especially among 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 has declined during the pandemic due to lack of exposure to the latest flu strains, more people than usual may be susceptible to the virus.

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

“Decreased natural immunity is a concern,” said Dr. Olsen, “and lower immunity could lead to more infections and more severe disease.”

The result could mean bigger outbreaks outside of the flu and RSV seasons, Dr. Baker said. In fact, in Florida, RSV would normally decline at this time of year, but it is currently on the rise.

If offices and schools reopen in large numbers in the fall, as many expect, scientists will be watching closely.

“We are always concerned about the flu that causes severe illness, especially in people at high risk for complications,” said Dr. Olsen. “We know that school-age children are important factors in transmitting the flu virus. However, because the flu is difficult to predict, we cannot predict the severity of next season. ”

There is also a positive potential in the absence of the flu: fewer cases usually lead to fewer mutations.

“At this time, because the flu is not circulating as much, the virus may not have had as much opportunity to evolve,” said Dr. Baker, “which means our vaccines may be more effective. than normal ”.

Choice of strains for influenza vaccine

Creating the flu vaccine this year has been more difficult than in the past.

Each year, scientists evaluate the flu strains circulating around the world and meet to decide which strains to protect against that year’s vaccine. They look at the strains that make people sick and use this information to predict which strains are most likely to infect people when the flu season sets in.

“We met in late February to make these recommendations,” Dr. Webby said, referring to the World Health Organization’s panel evaluating the flu vaccine. “It was difficult. The amount of data was smaller than the order of magnitude. ”

Dr. Olsen, the CDC epidemiologist, pointed out that vaccine choices are based on more than existing strains. Scientists are considering other data, including predictions of “the likelihood that any emerging group of influenza viruses will become more widespread in the coming months.”

And, she said, the uncertainty about the return of the flu makes flu vaccination more important, not less important.

There is another factor that is difficult to predict, which could play a significant role when the flu returns: whether society will continue to learn behaviors in the pandemic that benefit public health. Will masked clothing become the norm? Will employers give employees more physical space?

The last time Americans had the chance to make these behaviors part of the culture, Dr. Baker pointed out, they did not.

“The 1918 flu pandemic should have been something that gave us a kind of societal learning,” said Dr. Baker, but the behavior has not changed. “So what’s the journey you’re going to make since the Covid-19 pandemic along that axis?” she added. “Will you wear your mask, even if no one else is?”