MISSED SHOT

This winter’s awful flu season was made worse by a wildly ineffective vaccine

There’s no such thing as a mild flu season, Dan Jernigan, director of the US Centers for Disease Control’s influenza division, told reporters last month. Yet if the most common strain of the influenza virus is nasty every year, this season is arguably worse.

According to the latest report from the CDC, one out of every 14 US hospital visits during the week of Jan. 21-27 was because of flu-related symptoms, like persistent fever, chills, and muscle aches. There have been more than 102,000 cases confirmed through lab testing—about 20% of all the respiratory infection samples sent out. Fifty-three children have died from the virus.

A number of factors aligned this flu season, including the fact that the vaccine against the main strain of the flu is particularly ineffective. A paper from Canadian scientists published last week suggests that the vaccine is only 17% effective against H3N2, the strain that’s causing 80% of flu infections. Worse, in those aged 20 to 64, it’s only 10% effective.

This study, conducted in Canada, isn’t necessarily representative of the entire Northern Hemisphere that’s in the thick of flu season (it hits annually during winter in each hemisphere). Still, it’s the first examining the effectiveness of this year’s shot in practice. The CDC keeps track of estimates of the annual flu vaccine efficacy based on similar studies. (Some of the studies for the 2008-2009 and 2016-2017 season have not been published or peer-reviewed.)

Scientists predicted that this year’s flu shot wouldn’t be as good as others before it in part because of a mutation the virus developed during the genesis of the vaccine. Additionally, the H3N2 virus is the ‘problem child‘ of influenza, according to STAT News. The proteins on it that give the strain its numerical name are particularly virulent, especially among older populations.

You should still get a flu shot, this year and in years ahead. It boosts herd immunity, or the idea that if fewer people get sick from the flu, fewer people are capable of spreading it to others who may be especially vulnerable, like the elderly or children. And should you fall ill with the flu, definitely stay home and wait to get better.


Read next: For the first time, scientists found the flu can cause heart attacks

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