The U.S. influenza season is off to a fast start, which may hurt insurers, hospitals and people lulled by the milder outbreaks of the recent past.

The number of flu cases began to rise in November and reached an elevated level in December, a month earlier than usual, said Michael Jhung, a medical officer with the U.S. Centers for Disease Control and Prevention’s influenza unit. About 5.6% of all doctor visits in the U.S. now are for influenza, according to the Atlanta-based agency. That compares with 2.2% of visits at the peak of the season last year.

Companies most affected by the early start include insurers like Centene Corp., Coventry Health Care Inc. and Humana Inc., said Michael Manns, a Bloomberg Industries analyst in Princeton, N.J. They have the largest exposure in states such as Florida, Texas and Georgia, where the virus is widespread, indicating their medical costs could rise as a result.

“These companies have a larger percentage of their enrollment in states with a high incidence of the flu,” Manns said in a telephone interview. “If we look at those who are slanted toward older patients, we’ll see an increase in doctor office visits and hospital outpatient visits. That will probably drive utilization higher rate than people may have anticipated.”

Hospital companies including Health Management Associates Inc.  and Vanguard Health Systems Inc.  are likely to take a hit from the influenza outbreak because 93% of their beds are in the 25 states reporting elevated flu levels, Manns said. The spread of the virus will boost emergency room visits by the uninsured, which cuts revenue rates per patient, he said.

To contact the reporter on this story: Michelle Fay Cortez in Minneapolis at mcortez@bloomberg.net

To contact the editor responsible for this story: Reg Gale at rgale5@bloomberg.net