A flu vaccine protects for roughly one season and is then replaced with a different formulation. The virus changes faster than immunity lasts, and the production timetable forces a decision months ahead.
The virus changes its surface continuously
Immunity to influenza depends largely on recognising proteins on the surface of the virus.
Influenza replicates with frequent copying errors, and variants whose surface proteins differ enough to evade existing immunity have an advantage and spread.
This gradual accumulation of change is called antigenic drift, and it means the circulating population of viruses is never quite the same two years running.
Strains are selected long before the season
Global surveillance networks collect and analyse samples year round, tracking which variants are spreading and how they are changing.
Expert committees review that data and recommend which strains vaccines should contain, separately for each hemisphere, several months ahead of the season.
The lead time exists because manufacturing cannot be compressed, and it means the selection is a forecast rather than an observation.
Production methods set the timetable
Much influenza vaccine is still grown in fertilised chicken eggs, a process that requires enormous quantities of eggs and a fixed number of weeks.
Cell-based and recombinant methods have shortened parts of the process and removed dependence on egg supply, and they are used for a growing share of production.
Even so, filling, testing and distributing hundreds of millions of doses takes months, which is why the strain decision cannot wait.
Mismatch explains varying effectiveness
When the circulating virus resembles the selected strains, protection is stronger. When a variant emerges or becomes dominant after selection, protection falls.
Adaptation during egg-based growth can introduce further differences between the vaccine strain and the virus in circulation.
This is why effectiveness varies noticeably between seasons even when uptake and manufacturing are unchanged.
Partial protection still changes outcomes
Even in a poorly matched season, related immunity tends to reduce severity rather than eliminate infection.
Because influenza's most serious consequences fall on older adults, young children and people with chronic conditions, reducing severity is a substantial part of the benefit.
Vaccination also reduces onward transmission to some degree, which matters in settings such as care homes and hospitals where those groups are concentrated.
The annual cycle is therefore not a failure of the vaccine but a consequence of a virus that changes faster than any single formulation can follow.