How is duration of efficacy estimated for vaccines?

Question

Vaccines, especially those given in adulthood, usually have term limits attached, eg: 10 years for yellow fever or 3 years for typhoid. Since presumably the time course of an immune response is no great respecter of our calendrical conventions, and since there is also presumably a spectrum of responses across the population, how are these durations estimated? What are the criteria for deciding a cutoff time?

Also, do the estimates get revisited and updated as time goes on and new data become available? Do new data become available? Is there continuing follow-up research into this, or does the estimate just get made before the vaccine comes to market and then get taken as gospel?

Answer 1

Duration of efficacy is typically determined by tracking the antibody titers of a cohort of subjects who have gotten the vaccine, and estimating based on the trajectory of those titers where they will eventually cross the threshold to the point where the vaccine no longer confers immune resistance.

These estimates do get revised and estimated as time goes on - you will occasionally see new recommendations for a second or third "booster" dose of a vaccine, which is meant to extend the duration of immunity beyond the duration of the original vaccine.

Two major types of studies track this over time. Phase IV clinical trials, which are clinical trials required post-licensure, and observational epidemiology trials, which tend to be performed when disease transmission starts to occur in supposedly vaccinated populations.

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Mechanical snail has raised the issue of viral evolution, so I'll touch on it briefly. The duration of efficacy discussed above is based on how long a patient's body can mount an immune response to a particular challenge. That's a concern for all vaccines.

For some vaccines, there's a secondary process that's of concern - that of the virus evolving in such a way that the antigens targeted by the vaccine are no longer those on the virus itself. This is only a concern for some viruses, notably those that are particularly fast-evolving, like influenza or HIV, and less of an issue for say, measles and HPV.

But that's typically not what people are talking about when they say "duration of efficacy" because it's inherently unpredictable, and less a function of the vaccine and more a function of the virus.

Answer 2

I read somewhere (I don't have a citation) that duration of efficacy is limited by the evolution of viral antigens; eventually the population will evolve to the point where the immune system no longer recognizes them effectively. RNA viruses have more volatile genomes and so their antigens drift faster. This explanation makes some sense but it conflicts with @Epigrad's answer.