Young people and middle-aged adults were at high risk this flu season. Working-age adults accounted for 61% of influenza hospitalizations, the Centers for Disease Control and Prevention reported.
Last flu season, about 35% of flu hospitalizations were in people ages 18 to 64, CDC reported Thursday.
People in that age range accounted for about 60% of flu deaths. That compared with 18%, 30% and 47% for the three previous seasons, the CDC reported.
"One of the reasons flu is hitting young adults hard is such a low proportion get the flu shot," said CDC Director Tom Frieden. "Only one-third were vaccinated." That's in contrast to a 60% vaccination rate for seniors and more than 50% for children.
Vaccination rates have a clear link to illness. "We're seeing more of our cases in working-age adults," said Edward Belongia, an expert on flu vaccine effectiveness at the Marshfield Clinic Research Foundation in Marshfield, Wis.
People with underlying illness, especially obesity, diabetes and lung problems, were at highest risk. Only about 15% of adults hospitalized for the flu didn't have an underlying medical complication, said Anne Schuchat, who directs the National Center for Immunization and Respiratory Diseases.
Flu is usually most dangerous to the elderly. But this season, the dominant strain was H1N1, which emerged in 2009. The last time a virus resembling H1N1 circulated was in the 1950s. People born later had never been exposed to it and had no immunity.
When H1N1 first appeared in 2009, so much of the population was vulnerable that it triggered a global pandemic, notable for high hospitalization and death rates among the young and middle-aged.
The good news was that this season's flu vaccine did a good job. Being vaccinated reduced the chance of having to go to the doctor for the flu by about 60%, the CDC reported.
That effectiveness rate is comparable with previous years. The CDC recommends that everyone 6 months and older get an annual flu vaccine.
"This season, vaccinated people were substantially better off than people who did not get vaccinated," Frieden said.
This year's flu season isn't finished, the CDC said. There will probably be several more weeks of flu activity, especially in regions where activity started later such as the Northeast and West Coast.