This year’s lackluster flu vaccine has proven to be only half as effective as usual against the primary viral strain that is in circulation in the U.S. this season. It also turns out it hasn’t done so well in Europe, either.
 

The same mutated version of flu is circulating in Britain and other European countries, and only about 3 percent of those vaccinated have been protected against influenza, British health officials estimate, NBC News reports.
 

That's worse than in the U.S., where federal health officials say the vaccine reduced disease risk by 23 percent – far lower than the 40 percent protection it offers against the so-called H3N2 flu strain in a typical year. Overall, health officials estimate the flu vaccine is usually about 65 percent effective in preventing flu and reducing its severity among those who do catch the virus.
 

Flu viruses mutate year to year, which is why new flu shot formulations must be devised and administered annually. But this year, the H3N2 strain mutated after the 2014-15 flu vaccine was developed, making the shot less effective against it.
 

Because it takes months to make flu vaccines, it was too late to start over again.
 

"In a season dominated by early circulation of influenza A(H3N2) virus, we found the overall vaccine effectiveness in preventing medically attended laboratory-confirmed influenza in primary care was only 3.4 percent," Richard Pebody, of Public Health England, and colleagues wrote in the journal Eurosurveillance.