Preparing for the 2018-2019 influenza season.

The 2017-18 influenza season brought record numbers of cases in both the Northern and Southern hemispheres, resulting in high rates of influenza-related hospital admission and death. Estimated efficacy of flu vaccine was low against the most common circulating strain, influenza A (H3N2). Decreased efficacy may be due to changes in circulating virus or mutations that occur in the vaccine strain during the manufacturing process.