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[15]The Flu Vanished During Covid. What Will Its Return Look
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The Flu Vanished During Covid. What Will Its Return Look Like?

By [18]Keith CollinsApril 22, 2021
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Note: Figures reflect weekly totals of positive flu tests, from public
and clinical laboratories.·Source: Centers for Disease Control and
Prevention

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

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

After fears that a [19]“twindemic” could batter the country, the
absence of the flu was a much needed reprieve that eased the burden on
an overwhelmed health care system. But the 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 do not know when it will come back in the United States, but we
know it will come back,” said Sonja Olsen, an epidemiologist at the
C.D.C.

Experts are less certain about what will happen when the flu does
return. In the coming months — as millions of people return to public
transit, restaurants, schools and offices — influenza outbreaks could
be more widespread than normal, they say, or could occur at unusual
times of the year. But it’s also possible that the virus that returns
is less dangerous, having not had the opportunity to evolve while it
was on hiatus.

“We don’t really have a clue,” said Richard Webby, a virologist at the
St. Jude Children’s Research Hospital in Memphis. “We’re in uncharted
territory. We haven’t had an influenza season this low, I think as long
as we’ve been measuring it. So what the potential implications are is a
bit unclear.”

Scientists do not yet know which public health measures were most
effective in eradicating the flu this season, but if behaviors like
mask-wearing and frequent hand-washing continue after the coronavirus
pandemic is over, they could help to keep influenza at bay in the
United States.

Much also depends on the latest flu vaccines, their effectiveness and
the public’s willingness to get them. The recent drop in cases,
however, has made it difficult for scientists to decide which flu
strains to protect against in those vaccines. It’s harder to predict
which strains will be circulating later, they say, when so few are
circulating 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 had
peaked in February. Then schools closed, travel halted and millions
began working from home, and the number of new flu cases quickly
dropped to historic lows, even as the coronavirus surged.

Influenza vs. coronavirus

Flu cases dropped even as the coronavirus spread.

Note: Figures reflect weekly totals.·Sources: Centers for Disease
Control and Prevention (flu cases); [20]New York Times database of
reports from state and local health agencies (coronavirus cases)

And the decline has not been because of a lack of testing. Since late
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 exposure to influenza, scientists say, may
partially 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 with the coronavirus — the population has some
natural immunity to the flu, from years of being exposed to various
strains of the virus. People are susceptible to new strains of the flu
each year, but less so than they are to wholly unfamiliar viruses.

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

And influenza was not the only virus that disappeared over the last
year; there were also substantial drops in other respiratory illnesses,
including the respiratory syncytial virus, or R.S.V., which is the most
common cause of pneumonia in infants.

What will happen when the flu returns?

Influenza is a relatively common illness that has the potential to
become deadly, especially among young children, seniors and adults with
chronic health conditions. The C.D.C. estimates that the flu has killed
12,000 to 61,000 people a year since 2010.

If immunity to the flu declined during the pandemic because of the lack
of exposure to the latest flu strains, more people than usual may be
susceptible to the virus.

“Every year, anywhere between 20 to 30 percent of the population gets
its immunity sort of boosted and stimulated by being exposed to the flu
virus,” Dr. Webby said. “We are not going to have that this year.”

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

The result could mean larger and out-of-season outbreaks of the flu and
of R.S.V., Dr. Baker said. In Florida, in fact, R.S.V. would normally
be on the decline at this time of year, but it is currently having an
uptick.

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

“We are always concerned about influenza causing severe disease,
particularly in persons at increased risk of complications,” Dr. Olsen
said. “We know that school-age children are important drivers of
influenza virus transmission. However, because influenza is difficult
to predict, we cannot forecast the severity of next season.”

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

“Right now, because influenza isn’t circulating as much, it’s possible
the virus has not had as much opportunity to evolve,” said Dr. Baker,
“meaning our vaccines could be more effective than normal.”

Choosing the strains for the flu vaccine

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

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

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

Dr. Olsen, the C.D.C. epidemiologist, pointed out that the vaccine
choices are based on more than just existing strains. Scientists also
consider other data, including forecasts of “the likelihood of any
emerging groups of influenza viruses becoming more prevalent in coming
months.”

And, she said, the uncertainty around the return of influenza makes
getting vaccinated against the flu more important, not less.

There’s another hard-to-predict factor that could play a significant
role when the flu comes back: whether society will carry on behaviors
learned in the pandemic that benefit public health. Will mask-wearing
become the norm? Will employers give their employees more physical
space?

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

“The 1918 influenza pandemic should have been something that gave us
some sort of societal learning,” said Dr. Baker, but behavior did not
change. “So what is the journey you are about to go on from the
Covid-19 pandemic, along that axis?” she added. “Will you wear your
mask, even if no one else is?”

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