Influenza vaccines are reformulated every year and their effectiveness varies considerably between seasons. Both facts follow from the timetable on which the vaccine has to be made.
The virus changes continuously
Influenza copies its genetic material with frequent errors, and changes accumulate in the surface proteins the immune system recognises.
Small accumulated changes gradually make previous immunity less effective, a process described as antigenic drift.
Because those surface proteins are what a vaccine trains the immune system to recognise, drift steadily erodes the match between last year's vaccine and this year's virus. This is the reason the vaccine is reformulated annually while others are not.
Surveillance runs continuously across hemispheres
A global network of laboratories collects samples from patients year-round and characterises the viruses circulating in each region.
Because seasons alternate between hemispheres, what circulates in one during its winter provides early information about the other's coming season.
That data is reviewed at scheduled consultations where the strains for each hemisphere's next vaccine are selected, using both what is currently common and which lineages appear to be gaining ground.
Manufacturing sets the deadline
Traditional production grows virus in eggs, a process that takes months and cannot be compressed once started.
The strain decision therefore has to be made roughly half a year before the season it is intended to cover.
During that interval the circulating virus continues to drift, and a strain that becomes dominant late can be one the vaccine matches poorly. The mismatch is therefore a forecasting problem created by production time, not a manufacturing error.
Mismatch reduces protection rather than eliminating it
Immunity generated against a related strain still provides partial recognition, so protection falls rather than vanishing.
Reduction in severe illness and hospital admission tends to hold up better than prevention of infection, since the two depend on different parts of the immune response.
This is why effectiveness figures against any infection and against serious outcomes differ, and quoting one for the other misrepresents the result.
Why the composition changes and coverage matters
Each vaccine covers several influenza types and subtypes, and any of them may be updated in a given year depending on what surveillance shows.
Newer production methods that avoid egg-based growth shorten some of the lead time and remove certain adaptation changes that occur during egg culture.
Which vaccine is appropriate for a given person depends on age and health conditions, and that decision sits with a clinician or vaccination service rather than with a general rule.