The current flu vaccines don’t match the predominant circulating flu viruses, which means they may not do much to prevent infection, according to researchers.

But they are still likely to prevent severe illness.”From our lab-based studies it looks like a major mismatch,” Scott Hensley, a professor of microbiology at the University of Pennsylvania who led the study, told CNN.

It’s bad news for the vaccine, he said. Influenza vaccines protect against four different strains of the flu: H3N2, H1N1 and two strains of influenza B. Hensley’s study only covers H3N2, but that happens to be the main circulating strain.

The vaccine mismatch may help explain an outbreak of flu at the University of Michigan last month that affected more than 700 people. More than 26% of those who tested positive had been vaccinated against flu — the same percentage as those who tested negative. That indicates the vaccine was not effective in preventing infection.

Get the full story at cnn.com.