The Centers for Disease Control and Prevention announced that this year’s flu vaccine might not provide robust protection against H3N2, the dominant strain circulating as of mid-December.

H3N2 is typically associated with more severe illnesses and worse patient outcomes, said CDC Director Thomas Frieden, M.D. He emphasized that people still should be vaccinated, and that the shot should have “some effectiveness” against the newly altered strains.

His “single most important message” was that providers should be prepared with antiviral medications, particularly for people at high risk for flu complications due to diabetes, lung disease or other health conditions. The antivirals Tamiflu (oseltamivir) and Relenza (zanamivir) combat flu once a person has been infected, but only about one in six people with severe flu receives them, the CDC leader said.