Why the flu shot worked less than half the time during last year’s flu season

As the temperatures drop, many are bundling up in coats and hats to stay warm and healthy. Folks are also getting their flu shots to ward off illness, but a new report reveals doctors are concerned about the strength of these vaccinations.

During the 2014-2015 season, flu vaccine effectiveness was just 19 percent, according to the Centers for Disease Control and Prevention, and it rose to 47 percent during the 2015-2016 season.

While it worked 42 percent of the time last season, it was only 34 percent effective against H3N2, the viruses that were most prevalent last year.

Why is that?

recent study by researchers from the Icahn School of Medicine at Mount Sinai found the low performance was due to a mutation in the influenza A vaccine strain, which was likely caused by the egg-based manufacturing process most commonly used today to quickly produce large quantities of the vaccine.

To determine these findings, analysts examined the seed strains, created in chicken eggs, that are distributed to vaccine makers.

When they aligned the strains of the vaccines with the circulating viruses, which can evolve over time, they noticed the mutation did not allow the antibodies in humans and animals to bind to the flu viruses. In other words, there was a mismatch between the current virus and current vaccine.

“Influenza vaccine effectiveness can be low when there is a mismatch between vaccine strains and circulating strains,” the authors said. “Viral antigens included in influenza vaccines are routinely updated in an attempt to avoid antigenic mismatches.” But this isn’t always possible to dodge.

Furthermore, when scientists compared egg-based vaccines with cell-based vaccines formed by insects or mammalian cells, they discovered both humans and animals with the cell-based vaccine responded significantly better to the circulating viruses.

Scientists now have their eyes on the current season. “The 2017–2018 recommended H3N2 component is the same as 2016–2017,” the study read, which could unfortunately cause another mismatch.

That’s why they hope to take on further investigations that “address whether egg-adapted mutations constitute another form of antigenic mismatch that alters vaccine effectiveness in other influenza virus seasons,” the authors said.

But despite the recent findings, researchers noted last year's vaccine wasn't an overall fail. It prevented nearly 30 percent of hospitalizations, and it reduced outpatient visits by 42 percent.

Want to learn more about the experiment? Take a look here.

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