By Caroline Humer

NEW YORK Jan 24 (Reuters) - The bills for the flu - at epidemic levels in the United States this winter - are piling up.

The nation's three largest insurers, UnitedHealth Group Inc , WellPoint Inc, and Aetna Inc, have paid at least $100 million more than usual this year to cover doctor and hospital visits by people with the flu.

The total cost is expected to mount this winter as influenza runs its course but it should still be less than a severe outbreak like in 2009. In part, that is because the flu's intensity appears to be subsiding.

The Centers for Disease Control on Friday said that the number of flu incidents has fallen in some areas of the United States, but continues to spread in others. The U.S. government agency said 47 states reported widespread flu activity, and 26 states and New York City had reported high numbers of flu-like illnesses.

The percent of flu-related doctor visits fell for the second week in a row, especially in the South, Southeast, Midwest and New England, the CDC said.

To be sure, the costs of the flu to insurers is only a small part of the economic damage. Statistical analysis by the CDC in 2007 - the most recent analysis - put the total cost of a typical flu season at $87 billion. Those costs include direct medical expenses such as hospitalizations and projected lost earnings due to illness and death.

The flu can hurt the finances of workers, the companies they work for, and the overall economy. Tourism drops, hurting airlines and cruise ships; retail sales fall as shoppers stay home and business contracts as sales stagnate.

"Absenteeism is loss of productivity, but then there is the person who is like the walking wounded. They show up to work, but you wonder what they are really contributing," Bill Rosenberg, a director in PwC Human Resource Services said.

And the flu's challenges come as corporate America is less staffed than in previous epidemics, and many workers and businesses are still struggling to recover from October's Superstorm Sandy, which destroyed buildings and infrastructure on the East Coast.

Some companies are helped, of course. Procter & Gamble Co said on Friday it has sold more of its Vicks branded products, Puffs tissues and Zzzquil, a new version of NyQuil. Kimberly-Clark said on Friday that its healthcare and facial tissue businesses had picked up in January and was probably helped by the cold and flu season.

STICKING TO THE COUCH

Dave Shove, an equity analyst at BMO Capital Markets, believes that the flu has peaked based on Google Flu Trends, which uses Google search data to estimate flu activity. He expects the insurers to set aside about the same amount of money for flu costs during the next few months as they did during October, November and December.

Most of the costs to insurers are for plans in which they assume the risk, such as for individual and small business coverage, and in government insurance programs like Medicaid or Medicare if they receive a fixed amount to cover each person's health care costs.

Still, a trend in the United States where consumers use doctors and hospitals less has made this a proportionately less expensive flu season for the insurers than, say, in 2009, Shove said.

"A lot of people are not going to the hospital to get admitted ... They are just staying on the couch at home," he said.

WellPoint, which reported earnings on Wednesday, said that the flu cost the company about $10 million, or 2 cents per share, during the fourth quarter.

WellPoint did not estimate the flu's future costs, but cited it as one of several factors behind uncertainty about its outlook for 2013. It said its Medicaid insurance plans, the government program for the poor and disabled, had been hit, but its insurance business for employers had not. (In that business, companies cover the costs of employees' health bills through self insurance.)

Last week, UnitedHealth, the nation's largest health insurer, said that the flu this year cost $50 million more than a typical flu. And earlier this month, Aetna told investors that it expected to spend more than its usual budget for a flu season of $40 million to $50 million, but less than the $100 million it spent in 2009 when swine flu hit countries around the globe.

Aetna is due to report on its December ended quarter next week, followed by Cigna Corp and Humana Inc. 

(Additional reporting by Jessica Wohl and Sharon Begley)

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