Antibody decay is normal and expected

After vaccination, the immune system produces antibodies and memory cells. Circulating antibody levels peak within two to four weeks and then begin to decline. This decline is a normal biological process, not a failure of the immune system. For vaccines that produce very durable memory cells, antibody levels may fall but immune memory persists, enabling a rapid recall response on future exposure. For others, both antibodies and memory decline to the point where re-vaccination is needed to restore protection.

The kinetics of immune memory

Long-lived plasma cells in the bone marrow continue producing antibodies at a lower rate for years to decades after vaccination or infection. Memory B cells circulate in the blood and lymph nodes, ready to proliferate and differentiate on re-exposure to the antigen. The relative contribution of long-lived plasma cells and memory B cells to ongoing protection varies between individuals and between vaccines. This variability explains why some people maintain strong immunity for decades while others need boosters more frequently.

Pathogen evolution as a driver of booster need

For RNA viruses like influenza and SARS-CoV-2 that mutate rapidly, waning immunity is only one reason for booster recommendations. The other is antigenic drift: the virus has changed enough since the original vaccine was designed that the original antibodies bind less effectively to the new variant. Annual flu vaccines address this by reformulating against each season's predicted circulating strains. COVID-19 booster vaccines are updated for the same reason.