The decline in vaccine immunity is not an all-or-nothing phenomenon, however, and a return infection often leads to much less severe symptoms. To add to the confusion, two essential vaccines, that against tetanus and that against diphtheria, seem to have a longer longevity than that supposed.
Immunological memory
The question of why some vaccines protect for a few weeks – one thinks of the flu shot – and others for a lifetime, has been the work of researchers for a long time.
It depends mainly on the immunological memory which is not fully understood and it remains one of the major challenges of vaccination, according to the immunologist who heads the Human Vaccines Project, Wayne Koff.
The whole science of vaccines therefore tries to reproduce the exploits of the natural memory of the immune system. The idea is to imitate its ability to react on contact with an infectious agent, using harmless imitations of viruses or bacteria.
90 days only
But how long can this reaction last after vaccination? In a review of 11 recent studies on the sustainability of influenza vaccines published in 2018, the researchers concluded that the efficacy may disappear as soon as 90 days after vaccination.
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