Flu vaccines only 25% effective against worst strain

This year’s flu vaccines reduce the chance of getting the flu by about one-third but are just 25% effective against the nasty strain causing the most misery, preliminary estimates showed Thursday.

The findings, published by the Centers for Disease Control and Prevention (CDC), come as no surprise to flu experts tracking the worst influenza season in a decade.

In a typical season, vaccines prevent 40% to 60% of flu cases serious enough to send people to the doctor. This year, vaccines are preventing 36% of those illnesses, the report said. But they are preventing just 25% of illnesses caused by a type of influenza A called H3N2. That strain, which always poses a vaccine challenge, is behind three-quarters of verified flu cases so far this season, CDC said.

Flu experts were quick to say the vaccines remain worthwhile.

“We’ve got a good vaccine but not a great vaccine. It is modestly effective,” said Arnold Monto, a professor of epidemiology at the University of Michigan and a coauthor of the report. The report is based on data from 4,562 patients.

Monto noted that the 25% effectiveness against H3N2 is higher than the 10% reported earlier from Australia. Canadian researchers estimated a 17% effectiveness rate. All the numbers come with statistical hedging: Because they are based on fairly small groups of patients, the real numbers could be higher or lower.

The new data also contain a bright spot: Among children ages 6 months to

8 years old, vaccine effectiveness was higher, 59% overall.

In any case, this does not appear to be the worst recent year for overall vaccine effectiveness. That would be

2014-2015, when the CDC reported a

19% flu prevention rate and blamed a mismatch between the strains in the vaccine and the ones causing illness.