TUESDAY, April 15, 2014 (HealthDay News) -- The H1N1 flu was the predominant influenza strain in the United States this year, but it packed a lot less punch than in 2009 when it caused a worldwide pandemic, health officials report.
This season's flu virus didn't reach those pandemic proportions because of prior widespread exposure and also because of its inclusion in the current flu vaccine, experts said.
"This year, not only do we have a vaccine that works well, but millions of people have already been exposed to the H1N1 virus," said Dr. Michael Jhung, a medical officer in the U.S. Centers for Disease Control and Prevention's influenza division. Although H1N1 flu has circulated since 2009, "this is the first season since then we have seen it dominate," he added.
As the 2013-14 flu season winds down, the CDC is able to make some comparisons with previous flu seasons, although final figures aren't yet available.
As in 2009, H1N1 hit younger adults harder than the elderly.
Of the more than 8,000 influenza-associated hospitalizations reported so far this season, over 60 percent have been among people 18 to 64 years old, the CDC said.
The timing was different, too. "We had a bit of an early peak to the flu season," Jhung said. "We typically see the highest activity in January/February, but it happened this season about a month early."
Even though H1N1 was the most common flu strain, a "herd immunity" kept it from becoming a pandemic again, Jhung said.
For a virus to become pandemic, it must pass easily from person to person and have never circulated before so that people don't build immunity to it.
"The theory of herd immunity is that if you vaccinate enough people not everyone has to be immune in order to stop the disease spreading through a community," he said.
For instance, infants under 6 months of age cannot be vaccinated for flu, but if the people around them are vaccinated, the odds of the infant getting the flu are greatly reduced, Jhung said.
Also, the makeup of the current flu vaccine made it more effective than the vaccines of the last couple of years, Jhung said. "We have a preliminary estimate for vaccine effectiveness that's quite good -- 62 percent," he said.
That means the likelihood of getting the flu if you were vaccinated was cut by 62 percent. "In the previous two years, vaccine effectiveness was between 47 and 49 percent," Jhung said.
This year's flu vaccine protected people from H1N1 and other flu types, including H3N2 and a type of influenza B virus. Some formulations of the vaccine had an additional B strain, protecting people against four types of flu, Jhung said.
The CDC relies on hospital data to estimate adult deaths at the end of the flu season, and it's too soon to obtain those figures, Jhung said. Adult flu deaths in the United States usually range from 35,000 to 40,000 a year.
The agency does have up-to-date records for children, however. So far this season, 85 children have died from flu. By the end of March last year, 110 children had died of flu. Five states -- Arkansas, Louisiana, New Mexico, Oklahoma and Texas -- accounted for 29 of this season's pediatric deaths, according to the CDC.
The best way to protect yourself from the flu is by getting a flu shot, the CDC advises. The agency recommends everyone 6 months old and up get a flu vaccine every year. This year, about 40 percent to 45 percent of those who should get a flu shot did -- about the same as last year, Jhung said.
Although some flu illness is still being reported in the United States, overall flu activity is low, the CDC said.
For more information on flu, visit the U.S. Centers for Disease Control and Prevention.