(USA Today) - Outbreaks of influenza are getting an early start this year in part because of cold weather gripping much of the USA and a low efficacy associated with this year's flu vaccine.

It's still too early to say whether this winter will be a bad season for the flu, but epidemiologists in 36 states already have reported widespread influenza activity to the federal Centers for Disease Control and Prevention data released Friday. Twenty-one of those states show a high number of cases.

"It's just one of those years where the CDC is seeing that this strain of flu is only somewhat covered by the vaccine that was given this year," said Jennifer Radtke, manager for infection prevention at the University of Tennessee Medical Center in Knoxville. "They're seeing that it's anywhere from 10% to 33% effective, so any time there's a mismatch between the vaccine and the circulating strain of the flu, you're going to see more cases."

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Vaccine effectiveness varies from year to year though recent studies show that the flu shot typically reduces the risk of illness by 40% to 60 percent among the overall population when the circulating virus is matched closely to the vaccine virus, according to the CDC.

Peak flu activity in the U.S. usually occurs around February.

Because only a certain percentage of people with flu symptoms go to hospitals and get tested, it can be challenging to track the actual number of people affected, Radtke said. False negative results for flu tests are also common, so it's likely the number of people with the flu is much higher.

"It's not uncommon to see it this time of year," she said. "The last couple of years we didn't see it in December, but a few years back we did. Cold weather doesn't hurt, but we've had cold Decembers before and not had flu."

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