Officials say flu shots available this year should do a better job than last year's version, which turned out to be poorly matched against the most prevalent influenza virus around, resulting in a bad flu year for Americans.

In last year's flu season, the vaccine proved ineffective against a particular strain of the H3N2 virus dubbed the "Switzerland variant," resulting in the highest hospitalization rates for people 65 and older ever documented.

The vaccine now becoming available has been updated to better match that strain, says Thomas Frieden, director of the U.S. Centers for Disease Control and Prevention.

An analysis of the most common strains circulating in the U.S. this season found they are well-matched by this year's vaccine, he said, encouraging people to take advantage of the flu shots.

"Get vaccinated," Frieden said. "That's the best way to protect yourself, your family and your community against flu."

Last year, a mutation of the H3N2 strain identified as A/Switzerland/9715293/2013 emerged after the vaccine had been formulated, too late to be included, resulting in an overall effective rate for the flu shots of just 23 percent, compared with the normal 50 percent to 60 percent effectiveness in most years.

Of 199 flu specimens analyzed as this year's vaccine was being prepared, 118 of them were H3N2 viruses, and all should vulnerable to the new vaccines, the CDC is reporting.

The remaining variants should also be targeted by this year's vaccine, it said.

While "it is not possible" to predict exactly which flu strains will circulate most widely this fall and winter, "the best method for preventing influenza and its potentially severe complications" is to get vaccinated, the agency says.

That is especially important for people 65 and older, since viruses of the H3 type tend to make seniors sicker compared with the effects of other strains, Freiden said.

An increased amount of flu vaccine – 171 million doses – is being produced this year, the CDC says, with 40 million already distributed and available.

That's the most doses made available since 2009, the agency said.