This season’s flu vaccine is a good match for the virus strains in circulation, according to new data from the Centers for Disease Control and Prevention, explaining in part the lower severity of the disease this season.

In its weekly report on Thursday, the federal agency said the interim estimate of the flu vaccine’s effectiveness against the H1N1 influenza strain, which has been dominant in the United States this year, was 47 percent. For context, at this time last year the midseason effectiveness of the vaccine was just 36 percent. The interim estimate covers October through February.

This season, nearly 90 percent of the flu cases tested by the CDC are turning out to be H1N1, the milder variety. Ten percent of people are still getting the H3N2 version of the flu, the stronger version that dominated last season.

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The 47 percent effectiveness is “a solid number, and that has to do with the fact that we have a very good match with the dominant virus that’s out there this year,” said Dr. William Schaffner, an infectious-disease expert at Vanderbilt University and medical director for the National Foundation for Infectious Diseases. But that's not the whole story, he said.

“This data doesn’t tell us about how it's prevented complications, which is one of the main reasons we give flu vaccine,” he said. “Milder illness means that people are less likely to have complications that result in hospitalizations or even death.”

It’s difficult to assess the severity of the flu season while in the midst of it, but researchers believe that this flu season is milder in part because of the vaccine’s increased effectiveness.

So far, the flu season has not taken off like it did last year, when close to 50 million people caught flu and 80,000 died in the worst flu season in decades. Currently, the CDC estimates 11,600 to 19,100 deaths from Oct. 1 to Feb. 9 and noted that this was “a low-severity influenza season,” with fewer outpatient visits for influenza-like illness, fewer hospitalizations, and fewer deaths attributed to pneumonia and influenza, compared with recent seasons.

Every flu vaccine is a cocktail, aimed at either three or four of the most common flu strains. Flu vaccines must be reformulated every year, because flu viruses mutate constantly in a process called antigenic drift. This season’s flu shot contains four strains — H1N1, H3N2, Influenza A and Influenza B — in what’s known as a quadrivalent vaccine. Because both the H1N1 and the H3N2 viruses match the vaccine strains very well, those who got the shot are likely to be very well protected.

Since October, up to 17.8 million Americans have gotten the flu, many of whom are unvaccinated kids or older adults. Flu can be deadly in those who are already sick. In fact, this year’s flu vaccine is especially effective in children, at 61 percent. Kids generally respond better than adults to the vaccine because they have stronger immune systems.

Despite the vaccines' far from perfect effectiveness, it is still the best way to avoid the virus.

“I wish they had told us how many deaths have been averted. When someone tells me they had the flu, I respond ‘I’m so glad you’re still here to tell me about it,’” Schaffner told NBC News.