Final answer:
A vaccine from a previous year may offer some protection against similar strains the following year, but due to antigenic drift and high mutation rates in influenza viruses, annual vaccinations are typically required to maintain effective immunity against new strains.
Step-by-step explanation:
If you had a flu vaccine last year and the strain causing the outbreak this year is very similar, the lasting effect of the vaccine you took could offer some level of protection due to cross reactivity. However, because viruses such as influenza undergo frequent mutations in their surface molecules through processes like antigenic drift, the protective immunity obtained from last year's vaccine may not be fully effective against the new strain. Influenza vaccines need to be updated annually to match the prevalent strains, as the high rate of antigenic variation makes it challenging for the immune system to recognize new strains.
Vaccination works by exposing the immune system to noninfectious antigens, creating immune memory. This process may provide protection against the corresponding pathogen during future exposures. Because memory cells have a limited lifespan and viruses can mutate, booster vaccinations or annual flu shots are often necessary to maintain immunity.