The Centers for Disease Control and Prevention (CDC) has issued a health advisory regarding a drift of Influenza A (H3N2) viruses, which may possibly lead to a decrease in flu vaccine effectiveness.

Currently influenza activity in the U.S. is low; however, influenza A (H3N2) has been reported most frequently and has been detected in almost every state.

Of the influenza A (H3N2) viruses that were analyzed in the United States from October 1–November 22, 2014, 48% were found to be antigenically "like" the 2014–2015 H3N2 component, and 52% were found to be antigenically drifted. More than 50% drift is not a good sign and is an indication that the vaccine currently in use may be less than 50% effective.

In light of this development, the CDC is reminding healthcare professionals of the importance of using neuraminidase inhibitor antivirals (like Tamiflu) when indicated for prevention and treatment of influenza, as an adjunct to vaccination.

Though, vaccination has been found to provide some protection against drifted viruses, clinicians should continue to administer flu vaccines to those unvaccinated during this influenza season. Also, the vaccine protects against circulating strains that have not antigenically drifted from the vaccine viruses (eg, influenza A (H1N1)and  (H3N2) viruses).

 The two antiviral medications recommended for prevention or treatment of influenza are Tamiflu (oseltamivir) and Relenza (zanamivir).

When indicated, it is recommended to initiate treatment as soon as possible after influenza onset. The use of these antivirals can shorten the duration of fever and symptoms, reduce the risk of influenza complications, and reduce the risk of death among hospitalized patients.

Healthcare professionals are recommended to encourage all patients 6 months and older to receive the influenza vaccine for the 2014–2015 season. In addition, healthcare professionals should encourage all patients with influenza-like illness at high risk for influenza complications to seek immediate care to determine if antiviral treatment should be initiated.