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?