The flu season is getting worse, and U.S. health officials say it’s partly because the flu vaccine doesn’t protect against most of the spreading flu bugs.

The flu shot is a good match for only about 40 percent of this year’s flu viruses, officials at the U.S. Centers for Disease Control and Prevention said Friday.

That’s worse than last week’s report when the CDC said the vaccine was protective against roughly half the circulating strains. In good years, the vaccine can fend off 70 to 90 percent.

Infections from an unexpected strain have been booming, and now are the main agent behind most of the nation’s lab-confirmed flu cases, said Dr. Joe Bresee, the CDC’s chief of influenza epidemiology.

It’s too soon to know whether this will prove to be a bad flu season overall, but it’s fair to say a lot of people are suffering at the moment. “Every area of the country is experiencing lots of flu right now,” Bresee said.

This week, 44 states reported widespread flu activity, up from 31 last week. The number of children who have died from the flu has risen to 10 since the flu season’s official Sept. 30 start, including three in Mississippi.

Those numbers aren’t considered alarming. Early February is the time of year when flu cases tend to peak. The 10 pediatric deaths, though tragic, are about the same number as was reported at this time in the last two flu seasons, Bresee said.

The biggest surprise has been how poorly the vaccine has performed.

Each winter, experts try to predict which strains of flu will circulate so they can develop an appropriate vaccine for the following season. They choose three strains— two from the Type A family of influenza, and one from Type B.

Usually, the guesswork is pretty good: The vaccines have been a good match in 16 of the last 19 flu seasons, Bresee has said.

The vaccine’s Type B component turned out not to be a good match for the B virus that has been most common this winter. One of the Type A components turned out to be poorly suited for the Type A H3N2/Brisbane-like strain that now accounts for the largest portion of lab-confirmed cases.

Over the years, the H3N2 flu has tended to cause more deaths, Bresee said.

This week, the World Health Organization took the unusual step of recommending that next season’s flu vaccine have a completely different makeup from this year’s. The U.S. Food and Drug Administration is expected to make its decision about the U.S. vaccine next week.

H3N2 strains are treatable by Tamiflu and other antiviral drugs, but the other, H1N1 Type A strains are more resistant. Of all flu samples tested this year, 4.6 percent have been resistant to antiviral medications. That’s up from less than 1 percent last year.

“This represents a real increase in resistance,” Bresee said.

On the Net:

The CDC’s flu season update: http://www.cdc.gov/mmwr