Britannica
Sources used in this answer
Why can a mismatch occur between a seasonal influenza vaccine and circulating viruses?
Created from Britannica articles
using AI.
AI can make mistakes, so verify using Britannica articles.
influenza
A colored transmission electron micrograph showing influenza viruses (red) at the outer surface of a host cell.
A mismatch between a seasonal influenza vaccine and circulating viruses primarily occurs due to the rapid and constant evolution of the virus. This evolution happens through two main processes:12
- Antigenic Drift: This involves random genetic mutations that cause minor changes in the virus's surface proteins, known as antigens. These subtle changes alter the epitope—the specific part of the antigen that antibodies target—allowing the virus to potentially escape the immune protection provided by a previous vaccine or infection.3
- Antigenic Shift: This is a more dramatic genetic alteration that results in a new influenza A subtype. It often occurs through genetic reassortment, where a host is infected with multiple strains simultaneously, leading to the emergence of a virus with novel antigens to which the human population has little to no existing immunity.41
Because influenza viruses are constantly evolving, global health organizations must continuously monitor circulating strains to update vaccine formulations annually. When the circulating viruses change significantly after the vaccine has been developed, the resulting mismatch can reduce the vaccine's effectiveness.5236