U.S. health officials said the vaccine performed poorly despite being well-matched to flu viruses circulating worldwide. It offered some protection from mild seasonal flu in adults over 65, but did not prevent hospitalizations with severe flu viruses.

The Centers for Disease Control and Prevention (CDC) is investigating if previous flu shots might undermine more recent flu shots, or if there is a better way to check how long protection lasts.

Epidemiologist Jill Ferdinands of the CDC said in a statement:

While it is clear we need better flu vaccines, it’s important that we not lose sight of the important benefits of vaccination with currently available vaccines.”

Vaccines for most other diseases are at least 90% effective, but the flu is particularly challenging. Flu vaccines only averaged 46% effectiveness in all age groups in the last 10 flu seasons in the United States.

The problem is that flu viruses change quickly, so a vaccine that works in one flu season may not work the next. Each year, researchers guess which flu viruses will be circulating in the U.S. based on global data and create a new flu vaccine before winter.

Studies show flu shots were ineffective in seniors in the 4 out of the past 7 flu seasons. Last year’s flu season was moderately-severe, dominated by Type A H3N2 flu viruses that tend to cause more hospitalizations and deaths than other seasonal flu viruses.

Type A H3N2 was also dominant in the severe 2014-2015 flu season, when the vaccine was only 18% effective against this type of flu virus. The hospitalization-rate for seniors last winter matched the 2014-2015 flu season.

The CDC recommends flu shots for most people over 6 months old, but the flu is most dangerous for adults over 65 years old. There is a high-dose flu vaccine for seniors called Fluzone® that is supposedly more effective. The trade-off is a higher risk of side effects.

Source: Flu vaccine ineffective for people 65 and older last winter

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