Influenza Activity — United States, 2014–15 Season

While the influenza vaccine varies in how well it works, vaccination remains the best way to prevent influenza illness and its associated complications. Ongoing, timely influenza surveillance is needed to inform the selection of influenza vaccine viruses used for vaccine production. Although vaccine effectiveness was reduced this season against most H3N2 viruses, vaccination was still protective against vaccine-like influenza A (H3N2) viruses, influenza A (H1N1) viruses and influenza B viruses. The 2014-15 influenza season was an H3N2 predominant, moderately severe season overall, and especially severe for people 65 and older. Adults 65 and older had the highest hospitalization rates. Antigenic and genetic characterization showed that most of the circulating influenza A (H3N2) viruses were different from the influenza A (H3N2) component of the 2014–15 Northern Hemisphere vaccines, resulting in reduced vaccine effectiveness against circulating H3N2 viruses.