Just as the flu season is beginning to peak it's become evident that this year's vaccine is not well-matched to one of the strains circulating widely nationwide and increasingly in Minnesota, the state Health Department reported Thursday.

That means some people who got a flu shot could still get sick from the bug, but probably not as sick as they would have been without the vaccine, officials said.

The Minnesota Department of Health said influenza activity has increased in the past two weeks, with a number of reports from schools and long-term care facilities -- but the total number of cases has been about average.

Up until the beginning of this month, more than 90 percent of the cases in Minnesota were a good match for the three strains included in this year's vaccine. But now that's changing, said Kristen Ehresmann, immunizations manager for the Minnesota Department of Health.

An increasing number of the samples tested by the Health Department have not matched one of the strains in the vaccine. In the last two weeks between 20 and 50 percent of the cases reported to the Health Department are a strain that is not contained in the vaccine. That could increase. Determining just which strain it is involves more complex testing by the Centers for Disease Control and Prevention (CDC) and can take weeks.

Ehresmann said, however, that the flu shot is still worthwhile. It protects against two out of the three most common strains. And even those who are infected with the third type will be about half as sick as they would be without the shot, she said.

"A 50 percent level of protection is better than not having any protection," she said.

The Centers for Disease Control and Prevention said earlier this week that the flu shot is a good match for only about 40 percent of this year's flu viruses.

Nationally, the number of cases rocketed in mid-January as some new strains arrived. The Brisbane/10 strain is the big culprit, one first spotted in Australia late last winter, too late for scientists to include in this year's vaccine recipe even if they had predicted it would gain steam.

Ehresmann said that it's very similar to the new strain showing up in Minnesota now, though it will be several weeks before testing at the CDC will prove it, she said.

Flu viruses come in different strains that constantly mutate. Each year's vaccine contains protection against two varieties of the harsher Type A flu -- subtypes known as H1N1 and H3N2 -- and one from the more benign Type B family.

The CDC and international authorities expect Brisbane/10, a version of the H3N2 flu, to still be around next year. They predict a second new Type A strain, known as H1N1/Brisbane/59, also will hit, along with a newer Type B/Florida strain. That prompted decisions by federal and international health officials to put all three in next year's vaccine. Every year choosing the strains that go into the vaccine is a gamble based on tracking illness around the globe. The CDC has guessed right in 16 of the last 19 flu seasons.

This Associated Press contributed to this report. Josephine Marcotty • 612 673 7394