Flu shot effectiveness in question as worse-than-normal season looms for US

Signs are pointing to a worse-than-usual flu season this year and it’s too soon to tell whether the shot will be effective in preventing infection, experts say.

This year, the World Health Organization recommended that the Northern Hemisphere vaccines contain the same strains as those which were used during flu season in the Southern Hemisphere.

While this strategy sometimes works, it's possible the vaccine will end up being the wrong match for the virus that dominates in the United States.

“Australia, where flu season just ended, saw very high levels of influenza activity this year,” according to Nicole Bouvier, MD, associate professor of Infectious Diseases at the Icahn School of Medicine at Mount Sinai.

Preliminary estimates of vaccine effectiveness were low — overall 33 percent effective in preventing influenza and only 16 percent in preventing hospitalization with influenza.

“Those two things together suggest that our flu season may be worse than usual and/or the vaccine may be less effective than usual,” she said.

Typically, the influenza vaccine takes between six and eight months to produce and approve.

This means recommendations for vaccine strains have to be made in February or March for the following influenza season in the Northern Hemisphere and in September in the Southern Hemisphere.

By the time data is analyzed from the previous flu season in the Southern Hemisphere, the Northern Hemisphere vaccine is already being administered and it’s too late to change its makeup.

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“All of this is essentially speculation, albeit evidence-based speculation — the only thing predictable about influenza is its unpredictability,” Bouvier said.

So far, the U.S. is mostly being affected by the A(H3N2) strain of influenza - the same as that which impacted Australia.

The good news is that most of these viruses have been similar to the vaccine strain, meaning there are reasons to believe this year might not be a complete “vaccine mismatch,” according to Bouvier.

Experts say it could be weeks or months until it's clear whether the current vaccine is working well.

In the meantime, the Centers for Disease Control (CDC) continue to recommend the public gets vaccinated.

“It’s important to remember that the flu vaccine is designed to protect against three or four flu viruses, depending on the vaccine,” they said. “For these reasons, even during seasons when there is a less-than-optimal match, CDC continues to recommend flu vaccination.”

This is particularly important for high-risk individuals, including children under age 5, adults over 65 and pregnant women.