This season’s flu shot 45 percent effective, an improvement over last season’s vaccine

A first look at how well the flu vaccine is working suggests it’s better than last season’s shot — so far.

The Centers for Disease Control and Prevention reported Thursday that the overall effectiveness of the shot in the current flu season is 45 percent.

The CDC evaluates the annual vaccine on how well it prevents illness severe enough to send a person to the doctor or become hospitalized. This season’s shot has reduced the number of bad flu cases by a little less than half.

But, experts say, the overall effectiveness could change because we’re still in the middle of prime flu season, and other strains could pop up.

That’s what happened last season: The 2018-2019 flu vaccine ultimately turned out to be a poor match for the circulating viruses, being just 29 percent effective, thanks to a late-season surge of a particularly vicious flu strain.

This season, two main flu strains have been circulating: A/H1N1 and B/Victoria. When the CDC looked at how well the shot has been guarding against each strain, investigators found it’s been 50 percent effective against B/Victoria.

That was a welcome surprise to the CDC, because the B strain in the vaccine is actually mismatched against the B strain that’s circulating.

“We were concerned about the B virus,” Brendan Flannery, lead investigator for the CDC’s flu vaccine effectiveness network, told NBC News.

Perhaps more perplexing is that the preliminary data show protection against the A/H1N1 strain is lower, at 37 percent overall. That’s also a surprise, because experts said the vaccine was a good match for that strain of the virus.

And when the CDC looked more closely at the A/H1N1 statistics, they found virtually no protection was offered for adults ages 18 to 49.

“It’s really a mystery as to why we don’t see effectiveness against H1N1 in that adult age group,” Flannery said. He said cases of A/H1N1 continue to rise week to week, and it’s too soon to know for sure whether that age group will end up with any protection.