The bottom line is: "Data on influenza vaccines suggests that repeated vaccinations in an individual might ultimately result in a blunted immune response, decli

The bottom line is: "Data on influenza vaccines suggests that repeated vaccinations in an individual might ultimately result in a blunted immune response, declines in vaccine effectiveness, and a possibly reduced duration of protection... ...The underlying mechanisms through which previous vaccinations can affect the effectiveness of subsequent vaccinations in an individual are unclear, but two theories are widely discussed in the influenza literature. " The theories are: "...the antigenic distance hypothesis... the blunting of influenza vaccine effectiveness by previous influenza vaccination has been the most pronounced when the vaccine antigen was unchanged, but an antigenically distinct virus became the predominant circulating strain." And: "Since vaccine-induced immunity blocks or reduces the risk of infection, the theory asserts that the percentage of people who are unvaccinated with presumably stronger infection-induced immunity will increase over time. Thus, estimates of COVID-19 vaccine effectiveness will probably decline as the gap in immune protection between people who are repeatedly vaccinated versus those who are unvaccinated declines." The first theory would apply in instances where the antigen changes but the vaccine is not updated. The second theory would apply if vaccinated people didn't also acquire infection-induced immunity.