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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.

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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.

The news was slightly better for kids under age 18. Overall, the flu shot was found to be 55 percent effective in children and teens.

Early #fluvaccine effectiveness estimates for the 2019-20 season show that the #flu vaccine reduced flu-associated doctor’s visits by 45% overall, by 50% against flu B and by 37% against flu A(H1N1): https://t.co/zhrO7rlVDU pic.twitter.com/ZgofiCHWvh — CDC Flu (@CDCFlu) February 20, 2020

The CDC released the interim report in advance of a meeting scheduled next week in Geneva, where vaccine experts from around the world will discuss what influenza strains should be included in next season's flu shot.

So far this season, the CDC estimates there have been at least 26 million flu illnesses, resulting in a quarter-million hospitalizations and 14,000 deaths. Ninety-two children have died.

Dr. Alan Taege, an infectious diseases specialist at the Cleveland Clinic, said he continues to see a steady stream of patients with the flu. Even though the flu shot isn't 100 percent effective, he encourages people to get it every year.

"The more people you protect, the healthier the population is, and fewer people are going to have influenza to spread it to the rest of the population," he said.

All major medical groups in the U.S. recommend virtually all Americans age 6 months and older get the annual flu shot, saying that although it's not perfect, it remains the best way to protect against seasonal flu.

"Any protection that we can offer people to try to prevent as much of this as possible is well worth the effort," Taege said.

The CDC's Flannery said influenza was still circulating. "We're in the middle of the season, and we don't know how long it will continue."

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