This flu season is considered by the Centers for Disease Control and Prevention to be highly severe, but one age group is feeling it more than usual.
People ages 50 to 64 are particularly badly affected by this year's flu, the CDC told reporters Friday. That could be because of the strains in circulation. While the predominant strain is the nasty H3N2, which is less well-protected by the flu vaccine, H1N1 is also sending baby boomers to the hospital.
One possible reason why, according to the CDC's Dr. Dan Jernigan: a phenomenon called imprinting.
"The first influenza virus somebody's exposed to as a child has way of determining how you respond to influenza for the rest of your life," said Jernigan, director of the Influenza Division at the CDC's National Center for Immunization and Respiratory Diseases.
With H1N1 in 2009, for example, "we saw a strikingly low number of people being hospitalized over age 65," he said. That strain first emerged in 1918. "From 1918 to 1947, when that H1N1 was circulating, people that were exposed to that one seemed to respond better when this 2009 H1N1 showed back up," Jernigan explained.
Folks not imprinted by that strain, in the 50- to 64-year age group, therefore may not have the same protections as older people.
"There's still a lot for us to learn about this," Jernigan said, pointing out it "shows the complicated nature of influenza."
As for the predominant H3N2 strain in circulation this season, that first arose in 1968, Jernigan said. And it's continued to change, and evade the human immune system, since then.
"It really depends on a mixture of things: the environment, the virus itself and how it changes," and, he said, "also the host."
WATCH: Widespread flu outbreak causing serious complications this year
