The swine H3N2 virus, which includes the M gene of the 2009 H1N1 virus, has recently sickened 12 in five states -- Indiana, Iowa, Maine, Pennsylvania and West Virginia -- since August 2011.
First author Dr. Danuta Skowronski, a physician and epidemiologist with the British Columbia Center for Disease Control, said the study showed young adults had antibodies against the viruses, but antibody levels waned in middle-aged people. The study, published in Eurosurveillance, indicated the seasonal flu vaccine yielded little protection against the swine strains of flu.
Skowronski said the ferret virus used in the study was a surrogate for swine-related H3N2 viruses, including the novel strain.
"While it is not a precise match, it is useful for demarcating major age-related patterns expected with swine-origin H3N2 viruses, all of which are descended from a common ancestral human influenza virus that circulated about 15 years ago," Skowronski told the Center for Infectious Disease Research and Policy at the University of Minnesota.
Flu experts have said people older than age 20 should have some protection against the swine H3N2 viruses, because similar viruses were circulating in humans in the 1990s. However, this study suggested this might not be true for middle-aged people.
People older than age 59 were not included in the study, but the researchers said the age-related drop in protection raises concerns for seniors.
Skowronski said more work needs to be done to test the adult findings, which are preliminary.