How viruses and bacteria evolve and what this means for vaccine effectiveness over time
Pathogens are not static. Viruses mutate and bacteria evolve, sometimes producing variants that partially or fully escape immunity conferred by prior vaccination or infection. This is not a failure of vaccines — it is a fundamental feature of evolutionary biology that affects all immune responses.
Understanding pathogen evolution helps explain why some vaccines require regular updates, why effectiveness can change over time, and why ongoing surveillance is essential.
RNA viruses like influenza and SARS-CoV-2 mutate frequently because their replication machinery lacks error-correction. Most mutations are neutral or harmful to the pathogen, but occasionally a mutation provides a survival advantage — such as escaping immune recognition.
Variants that can infect immune individuals have a selective advantage — they can spread through a population with high baseline immunity. Over time, this selects for variants with immune-escape properties.
Antigenic drift is gradual accumulation of small mutations. Antigenic shift is a sudden major change, often from reassortment of genetic material between strains. Influenza undergoes both; shift can produce pandemic strains.
Public health agencies continuously sequence pathogen samples from infected individuals to track emerging variants. WHO coordinates global influenza surveillance through its GISRS network; similar systems operated for SARS-CoV-2.
Post-licensure effectiveness studies track whether vaccines continue to perform as expected against circulating strains. A drop in effectiveness signals potential immune escape and may trigger vaccine reformulation.
Pathogen evolution can effectively reset herd immunity calculations. When a new variant emerges with significant immune escape:
Andrews N, et al. "Covid-19 Vaccine Effectiveness against the Omicron Variant." New England Journal of Medicine. 2022;386:1532-1546. https://doi.org/10.1056/NEJMoa2119451
CDC. "Vaccine Effectiveness: How Well Do Flu Vaccines Work?" Centers for Disease Control and Prevention. https://www.cdc.gov/flu/vaccines-work/vaccineeffect.htm
WHO. "Global Influenza Surveillance and Response System (GISRS)." World Health Organization. https://www.who.int/initiatives/global-influenza-surveillance-and-response-system
Lauring AS, Andino R. "Quasispecies theory and the behavior of RNA viruses." PLOS Pathogens. 2010;6(7):e1001005. https://doi.org/10.1371/journal.ppat.1001005