Flu Vaccine May Not Protect Against Main Flu Strain

This season's flu vaccine may do little to prevent infection with the main circulating virus strain because the strain has changed since the vaccine was developed, a new study suggests. The study was posted on a preprint server online and has not been published in a peer-reviewed journal.

However, the vaccine is still likely to protect against severe illness and death, according to the U.S. researchers. They assessed the vaccine's effectiveness against the H3N2 strain. The vaccine is also meant to protect against H1N1 and two strains of influenza B.

Lab tests suggest a "major mismatch" between the vaccine and the new, mutated version of H3N2, study leader Scott Hensley, Ph.D., a professor of microbiology at the University of Pennsylvania in Philadelphia, told CNN.

That finding may help explain a flu outbreak at the University of Michigan last month that affected more than 700 people. More than 26 percent of those who tested positive had been vaccinated -- the same percentage as those who tested negative. That suggests the vaccine was not effective in preventing infection, CNN reported.

However, Hensley said the changes in the virus probably will not protect it against immune system T cells, so this season's flu vaccine is still likely to prevent severe disease and death. "Studies have clearly shown that seasonal influenza vaccines consistently prevent hospitalizations and deaths, even in years where there are large antigenic mismatches," Hensley and colleagues write in their report.