Immunity absolutely decays in many cases unless refreshed. For example, many vaccines require later booster shots. See also the strong evidence around a "boost

Immunity absolutely decays in many cases unless refreshed. For example, many vaccines require later booster shots. See also the strong evidence around a "boosting" effect from circulating chickenpox cases. Namely, that the availability of the chickenpox vaccine, by reducing the amount of environmentally-circulating chickenpox virus, is now preventing natural reinforcement of adults' immunity via asymptomatic encounters, and thus leading to more (painful, symptomatic) cases of shingles, especially among people in their 30s/40s: https://www.sciencedaily.com/releases/2015/08/150811103555.htm https://www.sciencedaily.com/releases/2015/08/150811103555.h... https://www.lshtm.ac.uk/newsevents/news/2020/adult-exposure-chickenpox-linked-lower-risk-shingles-does-not-provide-full https://www.lshtm.ac.uk/newsevents/news/2020/adult-exposure-... https://www.webmd.com/skin-problems-and-treatments/shingles/news/20180904/shingles-on-the-rise-among-younger-people https://www.webmd.com/skin-problems-and-treatments/shingles/...