The flu season is winding down and turning out to be one of the mildest in years, according to data from the U.S. Centers for Disease Control and Prevention.

One possible explanation is that the flu vaccine generally was well-matched to the circulating flu viruses, CDC officials said.

The CDC compares flu seasons by looking at adult deaths from the flu or pneumonia in 122 cities, and at reports of flu-related deaths in children. Both were down significantly this year compared with the severe 2007-08 season.

The flu causes 200,000 hospitalizations and 36,000 deaths annually in the U.S., according to official estimates. The elderly, young children and people with chronic illnesses are at greatest risk.

Health Canada says between 4,000 and 8,000 Canadians — mostly seniors — will die from pneumonia related to flu and many others may die from other serious complications of flu.

Record vaccination distribution

Vaccination is the best protection, health officials say. A record 146 million doses of flu vaccine were distributed for the 2008-09 season, although the CDC doesn't have data on how many people actually got them.

Flu vaccines are often between 70 per cent and 90 per cent effective. In the 2007-08 season, the vaccine was only 44 per cent effective. No such figure has been released for this year, but tests indicate the vaccine matches up well to at least two of the flu viruses going around.

The main kind of flu virus circulating this year was a Type A H1N1. Flu seasons in which an H1N1 predominates are generally milder than seasons when a Type A H3N2 does, said Dr. Alicia Fry, a CDC epidemiologist.

As of March 28, 60 per cent (4,264 out of 7,227) of strains detected were influenza A, according to the Public Health Agency of Canada's Flu Watch website. Most were H1N1 types, the National Microbiology Laboratory reported.

The Canadian agency doesn't compile a summary until the season ends.

In the U.S., this flu season was perhaps toughest on doctors.

In recent years, doctors routinely prescribed a drug called Tamiflu to flu patients. But this season's H1N1 strain has been resistant to that drug. Only a few years ago, CDC officials announced that H3N2 flu had become resistant to two other antiviral medications, rimantadine and amantadine.

Lately, doctors have had to use a patchwork of medications and hope for the best.

"It was more challenging" this year, said Dr. Bruce Ribner, an Atlanta infectious disease physician.