Why is influenza vaccination recommended every year?

Annual influenza vaccination is recommended primarily because influenza viruses undergo constant and rapid evolution through a process called antigenic drift.21

This process involves random genetic mutations that alter the virus's surface proteins, known as antigens. Because these mutations allow the virus to continually change and evade the immune system, the protection provided by a previous year's vaccine or a prior infection may no longer be effective against newly circulating strains. Furthermore, immunity generated by vaccination tends to fade over time.312

To address these changes, public health experts, including those from the World Health Organization, analyze global surveillance data each year to identify the strains most likely to circulate in the upcoming season. This information guides the development of an updated vaccine designed to match those specific strains, ensuring the best possible protection against severe illness, hospitalization, and death.24