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Minnesota's flu season is off to an unusual start, with a B strain of influenza causing most of the lab-confirmed cases so far rather than the A strain, which usually emerges first and does most of the damage.

B strains accounted for about 80% of the positive flu specimens obtained in clinics and hospitals and tested in the first week of December, according to data released Thursday by the Minnesota Department of Health. In the state's prior 10 flu seasons, A strains emerged first, and B strains barely registered until February.

"Typically, an A strain will peak and then you'll have B in the background and then you might see a second small wave of B," said Kris Ehresmann, the Health Department's infectious disease director. "But we haven't seen this."

B strains have historically spread faster among children, and the current data at least conforms to that pattern. In the first week of December, only two flu outbreaks were reported in long-term care facilities, such as nursing homes, but 13 were reported in schools. (Outbreaks are reported in schools when they are missing 5% of their students, or three elementary students from the same classroom, due to flu-like illnesses.)

And a quarter of the 176 flu-related hospitalizations so far in Minnesota have involved children, a number that would be closer to 10% in a typical season, Ehresmann said.

Seasons with more B cases of influenza tend to be milder, but infected individuals can expect the same symptoms — fever, fatigue, sore throat, and body aches.

No pediatric flu deaths have been reported so far this year. Two were reported in all of the last flu season. Health officials watch that number closely because flu-related deaths are most common in the elderly and people with chronic diseases. Deaths among children can signal a harsher season.

Vaccine remains plentiful, Ehresmann said, and specifically accounts for the B/Victoria strain that is currently spreading, along with other strains. Concerned parents can also seek out the FluMist nasal spray, she added.

Clinics that participate in the state's surveillance network are reporting that nearly 4% of patients are suffering flu-like illnesses — almost double the rate that clinics have reported at this time of year in the prior five flu seasons.

The lingering question is whether the A strains will emerge and spread at their normal historical levels — especially after the holiday season, when family and friends get together and swap germs — or whether the dominant B strain will mute their impact.

The novelty of this season also makes it difficult to predict whether it will be milder, Ehresmann said. "Because we don't know how those other strains may play out as the season progresses, I'd be reluctant to make a prediction."

Jeremy Olson • 612-673-7744