Curiosities: Vaccine contains three possible strains of flu

Q. How do public health officials determine which strain of influenza to create vaccines for each year?
A. This year’s influenza vaccine in the United States contains three strains of the influenza virus. Last March, experts at the U.S. Centers for Disease Control and Prevention (CDC) and the World Health Organization chose those strains based on the varieties of flu virus that were present at the end of North America’s flu season.