An analysis by the Trust for America's Health found 35.7 percent of U.S. adults ages 18-64 got vaccinated against seasonal flu last season -- the most recent period with available data.
By comparison, 56.6 percent of children ages 6 months to age 17 and 66.2 percent of seniors 65 and older were vaccinated. The U.S. Centers for Disease Control and Prevention in Atlanta recommends all Americans 6 months and older get vaccinated each year.
H1N1 is the most prevalent flu strain this season, which can disproportionately and adversely impact otherwise healthy children and young adults, the CDC says.
"The trend of low vaccination rates among younger adults is particularly troubling this year, when they are more at risk than usual for the effects of the H1N1 strain of flu that's circulating," Jeffrey Levi, executive director of Trust for America's Health, said in a statement.
The overall flu vaccination rates remain low in the United States. Forty-five percent of Americans got a flu shot during the 2012-13 season, an increase from 41.8 percent in the previous 2011-12 flu season, but far below the U.S. goal of 80 percent.
During the 2012-13 flu season, vaccination rates were highest in Massachusetts at 57.5 percent, and lowest in Florida at 34.1 percent. Twelve states had vaccination rates of 50 percent or higher: Delaware, Hawaii, Iowa, Maine, Maryland, Massachusetts, Minnesota, Nebraska, North Carolina, Rhode Island, South Dakota and Tennessee.
Three states had decreases in their vaccination rates from the 2011-12 to the 2012-13 season: Florida, Kansas and Wisconsin.