Influenza vaccines, also known as flu shots, are vaccines that protect against infection by influenza
viruses. New versions of the vaccines are developed twice a year, as the influenza virus rapidly
changes. While their effectiveness varies from year to year, most provide modest to high protection
against influenza. The United States Centers for Disease Control and Prevention (CDC) estimates
that vaccination against influenza reduces sickness, medical visits, hospitalizations, and deaths.
Immunized workers who do catch the flu return to work half a day sooner on average. Vaccine
effectiveness in those over 65 years old remains uncertain due to a lack of high‐quality research.