The story behind your seasonal vaccine
by Sarah Elizabeth Adler, AARP, September 24, 2019
En español | For most of us, flu season
begins each fall. For flu experts in the United States, however, it might as
well begin in February.
That's when scientists and researchers from
around the world gather for a meeting hosted by the World Health Organization
(WHO), where they spend days reviewing which strains of the flu virus have been
making people sick and decide which strains the next season's vaccine should
cover.
But when experts touched down in Beijing this
year, their goal — to recommend a vaccine formula for the Northern Hemisphere's
2019-2020 flu season — hit a roadblock.
A new variety of the flu virus known as H3N2 had begun to spread rapidly
in several countries. In the United States, it caused a second wave of illnesses late in a flu
season that had already seen the rise and fall of another strain entirely.
"That was something that we were
concerned about and wanted to incorporate into the Northern Hemisphere vaccine
… so we delayed the H3N2 decision by a month,” says virologist and physician
Kanta Subbarao, director of the WHO Collaborating Centre for Influenza in
Melbourne, Australia, one of six WHO centers that help track flu worldwide.
The extra month gave experts more time to
analyze the newly circulating virus. By March, they had updated their
recommendation to include a suitable H3N2 strain for this season's flu shot,
which is now available.
More importantly, the delay proved an old
truth about an ancient virus: The flu is a moving target, and for the network
of laboratories, agencies and manufacturers who help make the flu vaccine each
year, tracking it is only the start.
A needle in a haystack
The challenge starts with the virus itself.
An expert shape-shifter, the flu is constantly
changing — mutating as it replicates itself — in ways that allow its strains to
get past our body's immune defenses even if we've had the flu before, or if we
roll up our sleeves for the shot each fall.
The result? “It's a bit of a war between us
and the virus,” says David Wentworth, director of the WHO Collaborating Centre
for Influenza in the U.S., which is run out of the Centers for Disease Control
and Prevention (CDC) in Atlanta.
This battle plays out not only within the
bodies of people who come down with the flu's signature fever, chills and
muscle aches, but also in laboratories around the world, where researchers must
work quickly to analyze how the flu virus is changing in order to predict what
it might do next.
The worldwide system of surveillance starts
when specimens from sick patients are sent to the lab for testing. Of those,
about 7,000 end up at the laboratory run by microbiologist John Barnes, who
leads the CDC's influenza genomics team.
"We're always busy, and we're always
getting new viruses to work on,” Barnes says. He and his team perform
year-round genetic sequencing to determine how flu viruses are behaving, both
in terms of which strains are infecting people and other characteristics, like
whether a specimen shows signs of resistance to the antiviral drugs that can
treat the flu.
From there, about one-third of the CDC
specimens will undergo further assessment as part of a labor-intensive process
known as antigenic testing.
“Almost every season's a little different, and
the viruses that circulate have nuance,” Wentworth says. “We're looking for a
needle in a haystack.”
Making the flu shot
The pressure also is on for U.S.
manufacturers, who have about six months after that first February meeting to
prepare, test and ship more than 100 million doses of the vaccine, which start
to show up at doctors’ offices and drugstores across the country by August.
In Swiftwater, Pennsylvania, the process
begins with a chicken egg — or rather 1 million of them. They arrive every day
during flu shot production season at the plant owned by Sanofi Pasteur, the
world's largest manufacturer of flu vaccines.
The fertilized chicken eggs are injected with
the viruses that have been selected for that year's vaccine, then left to
incubate for several days before the virus is extracted and inactivated for use
in the flu shot.
"It may sound like an old-fashioned
technology. It isn't,” says physician Monica Mercer, the company's director of
scientific and medical affairs. Today, around 95 percent of all flu vaccines
made in the United States start out in eggs, even as manufacturers look to new
technologies, like growing the virus in cell cultures, as an alternative.
One vaccine innovation that's already widely
available: flu shots formulated specifically for older adults, who scientists
have long known tend to respond poorly to the vaccine for the same reason they
are more susceptible to the flu in the first place — the weakening of the
immune system as we age.
Pigs, pandemics and a better vaccine
But even the most effectively formulated
seasonal flu shot is, by design, only intended to protect people from the types
of flu that commonly circulate in humans.
Among researchers, another main area of
concern is the possibility of a flu pandemic, which occurs when a flu virus
that typically infects animals, like pigs or birds, changes in such a way that
it can spread rapidly — and perhaps lethally — from person to person.
That's where experts like veterinarian and
professor Andrew Bowman, who leads Ohio State University's animal influenza
research program, come in.
Bowman and his team spend each summer
traveling to more than 100 county fairs, where they swab the snouts of
exhibition pigs in search of flu strains with pandemic potential. Much like the
CDC researchers who analyze human flu specimens, their goal is to use genetic
testing to get an early look at potentially troublesome changes in the viruses they
capture.
"As we characterize these strains, we can
identify viruses that might be able to spread human-to-human so that we can
have effective countermeasures in place before ... an outbreak,” Bowman says.
These countermeasures include collaborating with the CDC to have the building
blocks of a pandemic flu vaccine at the ready, should the need arise.
But it doesn't take a pandemic for the flu to
be a serious, even fatal, health threat — especially for older adults.
"The typical patient that gets complications
from the flu is usually an older person, and usually somebody with an
underlying medical condition,” says emergency physician Jeremy Brown, author
of Influenza: The Hundred-Year Hunt to Cure the Deadliest Disease in
History.
When to get your flu shot
It takes about two weeks to build up immunity
after receiving the flu shot, so experts say early vaccination is key: The CDC
recommends getting the flu shot early in the fall, by the end of October.
If you're 65 or older, ask your doctor about
the high-dose or adjuvanted version of the vaccine, which is formulated
specifically for older adults.
According to the CDC, an estimated 90 percent
of flu-related deaths and 50 to 70 percent of all hospitalizations for flu
occur among people age 65 and older. While the effectiveness of the flu shot
varies from year to year, the agency estimates that it prevents millions of
cases of flu and flu-related doctor visits annually.
Still, Brown notes, there is room for
improvement. Typically, the flu shot is about 50 percent effective, but that
rate has dipped as low as 19 percent in recent years. In those cases, he says,
low rates might be the result of a vaccine formula that isn't well matched to
the viruses that end up going around, despite the experts’ best prediction.
They might also be caused by a vaccine that, although well matched, just
doesn't stimulate our immune systems like it should.
One hope, he says, is that a universal flu
vaccine that protects against all possible strains of the virus could someday
replace the yearly shot entirely. But despite early trials, he says the
prospect is still a long way off.
In the meantime, the flu virus continues to
change, evolving in ways both expected and unforeseen. The expert advice, on
the other hand, remains the same: Get the flu shot. Your future self might
thank you.
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