Three reasons: 1. Influenza viruses mutate over time, 2. the virus strains circulating can vary year to year, and 3. our immunity wanes over time. That’s why the flu vaccine formula needs to be updated yearly, and we need to get a flu shot each season to be well-protected. Influenza viruses have proteins on their surface: hemagglutinin (HA) and neuraminidase (NA). Read more…