Unlike most vaccines, the seasonal influenza vaccine is rebuilt annually. The reason lies in how the virus changes and in how long manufacturing takes.
Surface proteins drift continuously
Immunity to influenza is directed largely at two proteins on the virus surface, hemagglutinin and neuraminidase. Antibodies recognize specific shapes on those proteins.
The influenza genome is copied by an enzyme without proofreading, so mutations accumulate with every round of replication. Most are inconsequential, but some alter the recognized shapes.
Variants that escape existing antibodies have an advantage in a population that has already been exposed, so escape variants are selected for over successive seasons.
A separate mechanism produces larger jumps
Influenza A carries a segmented genome. When two different viruses infect the same cell, whole segments can be exchanged as new particles assemble.
That reassortment can swap an entire surface protein at once, producing a virus that population immunity does not recognize at all.
The gradual process is called antigenic drift and the abrupt one antigenic shift, and they carry very different public health implications.
Strain selection runs on surveillance
A global network of laboratories collects and characterizes circulating influenza viruses year-round, tracking which variants are spreading and how they respond to existing antibodies.
Consultations use that data to recommend which strains each hemisphere's vaccine should contain, with separate decisions for the northern and southern seasons.
Because seasons in the two hemispheres are offset, activity in one provides partial early information about what may circulate in the other.
Manufacturing lead time forces an early decision
Producing enough doses for a country the size of the United States takes months, whether the platform is egg-based, cell-based or recombinant.
Selection therefore happens well before the season begins, which means the decision is a forecast rather than a match to what is actually circulating in December.
When circulating viruses drift after selection, the match is poorer and effectiveness for that season is lower, without the underlying approach being wrong.
Why protection also fades within a season
Antibody levels rise after vaccination and then decline over the following months, a normal feature of the immune response rather than a defect of the product.
That decline interacts with the timing of the season, which is one reason timing of vaccination is discussed in public health guidance each year.
Questions about individual timing, eligibility and the different formulations available are decided with a physician or pharmacist, since medical history and age both bear on the answer.