The U.S. Centers for Disease Control and Prevention notified doctors Wednesday night in a health alert that the recipe for this year’s flu vaccine would have decreased effectiveness. The federal agency issued the alert to re-emphasize the importance of prescribing antiviral medications for treatment of the flu.

“Vaccination has been found to provide some protection against drifted viruses,” the agency said in the alert. “Though reduced, this cross-protection might reduce the likelihood of severe outcomes such as hospitalization and death. In addition, vaccination will offer protection against circulating influenza strains that have not undergone significant anitgenic drift from the vaccine viruses.”

The virus of concern is influenza A (H3N2) – one which strikes older people, very young children and people with certain chronic health conditions with higher rates of hospitalizations and deaths. Monitoring from Oct. 1 to Nov. 22 has revealed that 48 percent of the H3N2 virus samples collected matched the vaccine, but 52 percent were antigenically different or had drifted.

The CDC advised doctors to consider prescribing Tamiflu and Releza, which have been shown to reduce severe outcomes of the flu when used as soon as possible after the onset of illness.