WEDNESDAY, April 8 (HealthDay News) -- As the flu season winds down, experts say this has been the mildest season in years.

Less severe strains of influenza and a good vaccine match for the strains that were circulating combined to create a milder season this year than last, according to the U.S. Centers for Disease Control and Prevention.

"If we look at mortality and the rate of hospitalizations, it seems like this year is less severe compared to last year and more similar to the years prior to last year," said Dr. Alicia M. Fry, a CDC epidemiologist. "The flu did not reach an epidemic threshold this year."

Historically, she explained, in years where the influenza type A H3N2 subtype is the predominate virus, the season is more severe. "This year was not one of those years," she said. "It was a year where the influenza A H1N1 virus was the predominate virus, followed by the influenza type B viruses."

The CDC arrived at this conclusion using data from 122 cities on deaths from flu or pneumonia among adults and flu-related deaths among children. It appears that flu-related hospitalizations and deaths were significantly lower this year, Fry said

Typically, the flu causes 200,000 hospitalizations and 36,000 deaths a year, according CDC estimates. The old, the very young and people with chronic illnesses are at greatest risk.

So far this flu season, 43 children have died from the flu compared with 68 during last year's flu season, according to the CDC.

Flu vaccines are often 70 percent to 90 percent effective. Last flu season, the vaccine was only about 20 percent effective against the H3N2 strain and less than 2 percent effective against the B strains, according to the CDC.

But this year's flu vaccine was a very good match for influenza A H1N1 and H3N2, Fry said.

And that's good news, because there had been concerns about antiviral resistance, she said. The drug Tamiflu (oseltamivir), routinely prescribed to people with the flu, is resistant to this year's H1N1 strain, and the H3N2 flu strain is resistant to two other antivirals, rimantadine (Flumadine) and amantadine (Symmetrel).

Although 146 million doses of flu vaccine were distributed this season, the number of people who were actually vaccinated is unknown, Fry said.

Cases of the flu started to increase in January and peaked in the middle of February, Fry said. "There has been decreasing activity since," she said. "However, we still have many states that are still seeing flu activity."

And what might occur next year remains a mystery. "Never predict the flu season," Fry said. "That's the secret."

Dr. Marc Siegel, an associate professor of medicine at New York University School of Medicine in New York City, said that several factors combined to make this year's flu season milder.

"The prevailing strains are not that virulent and are not particularly new," Siegel said. "The H1N1 strain is a distant descendant of the Spanish flu, but we have all built up a lot of immunity to it over the years."

In addition, he said, there was a lot of vaccine available this year, and there has been a high level of compliance. "Adding the 5- to 18-year-old age group to those who get vaccinated helped, since flu super-spreaders are generally children who don't take precautions, like washing their hands," he said.

"People are more aware of the flu because of recent media attention, but the former hysteria may have finally been converted into proper precaution-taking," Siegel added.

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