The flu disappears during the pandemic. What will its return look like?

There were fewer flu cases in the United States during this flu season than in any recorded. 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 during the same period was about 206,000.

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

After the fear that a ‘twindemie’ could overcome the country, the absence of flu was a much-needed postponement that eased the burden on an overwhelming healthcare system. But the lack of exposure to the flu could also make the population more susceptible to the virus when it returns – and according to experts, its return is certain.

“We do not know when it will return to the United States, but we do know that it will return,” said Sonja Olsen, an epidemiologist at the CDC.

Experts are less sure about what will happen if the flu comes back. In the coming months – as millions of people return to public transport, restaurants, schools and offices – flu outbreaks may be more common than usual, they say, or may occur at unusual times of the year. But it is also possible that the virus that returns is less dangerous, as it did not have the opportunity to develop while it was interrupted.

“We do not really have an idea,” said Richard Webby, a virologist at the St. Jude Children’s Research Hospital in Memphis, said. ‘We are in an uncharted area. We have not had such a flu season yet, I think as long as we have measured it. So what the potential implications are is a bit unclear. ”

Scientists do not yet know what public health measures were most effective in eradicating the flu this season, but if behaviors such as masking and regular hand washing continue after the coronavirus pandemic is over, they could help keep flu in the United States at bay. to keep State.

Much also 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 influenza strains in the vaccines should be protected. It is more difficult to predict which tribes will circulate later, they say, if there is so little circulating now.

What happened to the flu?

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

And the deterioration was not due to a lack of testing. Since the end of September, 1.3 million samples have been tested for influenza, more than the average of about one million in the same period last year.

According to scientists, the public’s history of exposure to influenza may partly explain why influenza virtually disappeared while the coronavirus was still spreading after safety measures were put in place.

“For something like Covid, where you have a fully susceptible 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 the coronavirus – the population has a degree of natural immunity to influenza, as it has been exposed to various virus strains for many years. People are susceptible to new flu strains every year, but less so than to completely unknown viruses.

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

And flu was not the only virus that disappeared in the past year; there has also been a significant decrease in other respiratory diseases, including the respiratory synthesis virus, or RSV, which is the most common cause of pneumonia in infants.

What will happen if the flu comes back?

Influenza is a relatively common disease that can be fatal, especially among young children, the elderly 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 the immunity to influenza decreases during the pandemic due to the lack of exposure to the latest flu strains, more people may be susceptible to the virus than usual.

“Every year, between 20 and 30 percent of the population, its immunity is strengthened and stimulated by being exposed to the flu virus,” said Dr. Webby said. “We’re not going to have it this year.”

“Decreases in natural immunity are worrying,” Dr. Olsen said, “and lower immunity can lead to more infections and worse diseases.”

The result could mean bigger and out-of-season outbreaks of flu and RSV, said dr. Baker said. In Florida, RSV will normally decline at this time of year, but it is currently on the rise.

If offices and schools start reopening in larger numbers in the fall, as many expect, scientists will be keeping a close eye on them.

“We are always concerned about flu causing serious illnesses, especially in people at increased risk for complications,” Dr Olsen said. ‘We know that children of school age are important drivers of the transmission of influenza virus. However, because flu is difficult to predict, we can not predict the severity of next season. ”

The absence of influenza is also possible: fewer cases usually lead to fewer mutations.

“Because influenza does not circulate as much, it is possible that the virus did not have as much chance of developing,” said Dr. Baker said, “which means our vaccines may be more effective than normal.”

Select the strains for the flu vaccine

It was harder to create the flu vaccine this year than in the past.

Each year, scientists evaluate the flu strains circulating around the world and gather to decide which strains in that year’s vaccine should be protected. They look at the strains that make people sick, and use the information to predict which strains infect most people when flu season arrives.

“We met at the end of February to make these recommendations,” said Dr. Webby said, referring to the World Health Organization panel evaluating the flu vaccine. “And it was annoying. The amount of data was less large than usual. ”

Dr. Olsen, the CDC epidemiologist, pointed out that the vaccine choices are based on more than just existing strains. Scientists are also considering other data, including predictions about ‘the likelihood that emerging groups of flu viruses will become more prevalent in the coming months’.

And according to her, the uncertainty surrounding the return of flu is more important, not less.

There is another difficult predictable factor that could play an important role if the flu comes back: whether society will learn behaviors that are taught in the pandemic and that are to the benefit of public health. Will mask wear become the norm? Do employers give more real space to their employees?

The last time Americans had a chance to make this behavior part of the culture, dr. Baker pointed out, they did not.

“The flu pandemic in 1918 must have been something that gave us a kind of social doctrine,” said Dr. Baker said, but behavior did not change. “What is the journey you are going to take on the point from the Covid-19 pandemic, along that axis?” she added. “Will you wear your mask, even if no one else is?”