By Rob Stein, NPR
We may be in for a nasty flu season. That's the warning out today from the federal Centers for Disease Control and Prevention.
The CDC is worried because the most common strain of flu virus circulating in the United States is one called H3N2. In previous years, H3N2 strains have tended to send more people to the hospital than other strains — and cause more deaths, especially among the elderly, children and people with other health problems.
Another concern is that more than half of the H3N2 viruses tested so far this year have "drifted," meaning they have mutated slightly from the strain used to make this year's flu vaccine.
"In past season during which predominant circulating influenza viruses have been antigenically drifted, decreased vaccine effectiveness has been observed," the CDC wrote in an alert sent to doctors.