Perhaps armed with the knowledge that they are the least vulnerable age demographic to contract the flu, 18 to 64 year olds all over the country said “no” to the flu vaccine during the 2012-2013 flu season, according to a report from the nonprofit organization Trust for America’s Health (TFAH).

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(MORE: This Year’s Flu Seasons Hits with ‘Dramatic Cases’)

But a flu season dominated by the H1N1 strain (swine flu) has lead to an irregular numbers of flu cases in otherwise healthy young adults, making this age group more vulnerable than most years, according to the U.S. Centers for Disease Control and Prevention.

“Flu season is just hitting,” Bruce Hirsch, M.D., infectious disease specialist at North Shore Long Island Jewish Hospital in New York, recently told weather.com. “It seems to be a more dramatic onset, and it’s affecting healthier people than usual.”

An analysis found that only 35.7 percent of adults ages 18 to 64 years old received the flu shot last year. Approximately 56 percent of children over the age of 6 months, and 66 percent of seniors received the vaccine.

Nearly 10 percent fewer adults received the vaccine last year than the year before (45 percent).

 “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,” said Jeffrey Levi, Ph.D., executive director of TFAH, in a press release. “It’s easy to become complacent about the flu.  We’re used to it, it happens every year.  So much so that we forget that it is largely preventable through a quick shot.”

The combination of vaccine refusal and a dominating strain that targets healthier individuals than usual could contribute to a severe 2013-2014 flu season.

“The flu isn’t just an uncomfortable inconvenience, it is deadly and costly,” added Dr. Levi.

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