Why are booster shots necessary for some vaccines?

Short version
Booster shots are given after a primary vaccine series to “remind” the immune system so protection stays high. They raise antibody levels and strengthen immune memory so you’re less likely to get sick or spread the infection.

Why boosters are needed (plain language)
- Immunity can wane: antibody levels and some protection drop over months or years after the initial vaccination. A booster restores those levels.
- Some vaccines induce only a short-lived response: inactivated or subunit vaccines often produce less durable immunity than live vaccines, so they need periodic boosts.
- The pathogen can change (antigenic drift): when viruses mutate (e.g., influenza, SARS-CoV-2), updated boosters can better match circulating strains.
- Certain people respond less well: older adults and immunocompromised people may need extra doses to reach or maintain protection.

What a booster does biologically (brief)
- Rapidly increases circulating antibodies that neutralize the pathogen.
- Re-activates memory B and T cells, promoting further affinity maturation and formation of long-lived plasma cells for stronger, longer-lasting immunity.
- Sometimes broadens the immune response, improving protection against variants (especially if the booster antigen is updated).

Examples
- Tetanus: routine boosters every ~10 years because immunity wanes.
- Influenza: annual vaccination is recommended because the virus changes each year.
- COVID-19: boosters have been recommended when immunity wanes or when new variants reduce vaccine effectiveness.

Practical note
Follow your local public-health or clinician recommendations for booster timing. If you’re older, immunocompromised, or have specific travel/work exposures, you may need additional or earlier boosters.