As in previous years, H1N1 disproportionately affected younger people -- nearly 60 percent of the 9,635 confirmed flu-related hospitalizations occurred in people between the ages of 18 and 64 years. And, people between the ages of 50 and 64 years had the highest rates of flu-related hospitalizations this year compared to the past four flu seasons, the CDC said.
These latest figures are from the June 6 issue of the CDC's journal Morbidity and Mortality Weekly Report.
Things could've been much worse, though. The predominant virus didn't cause a pandemic this year because of prior widespread exposure to H1N1, and also because of its inclusion in this year's flu vaccine, the CDC said.
The agency is recommending inclusion of the H1N1 virus in next year's flu vaccine as well.
"This year, not only do we have a vaccine that works well, but millions of people have already been exposed to the H1N1 virus," Dr. Michael Jhung, a medical officer in the U.S. Centers for Disease Control and Prevention's influenza division, told HealthDay. Although H1N1 flu has circulated since 2009, "this is the first season since then we have seen it dominate," he added.
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 noted.
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, 96 children have died from flu. In 2009, when H1N1 first surfaced, 348 children died from influenza, 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 to 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 the peak of flu has passed, flu activity continues throughout the summer, according to the CDC.
For more information on flu, visit the U.S. Centers for Disease Control and Prevention.