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 andPrevention. In recent years, the average number of cases over the same period
was about 206,000.
اضافة اعلان
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 “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 Centers
for Disease Control and Prevention (CDC).
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, Tennessee. “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.
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 1 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 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 (RSV), 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 CDC 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 percent to 30 percent of the
population gets its immunity sort of boosted and stimulated by being exposed to
the flu virus,” Webby said. “We are not going to have that this year.”
“Decreases in natural immunity are a concern,” Olsen said, “and
lower immunity could lead to more infections and more severe disease.”
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 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 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.”
Olsen, the CDC 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.
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