In the context of mRNA vaccine if you feel good 1 month after getting the mRNA vaccine or if you feel okay after 1 year means nothing because biological processes are slow but when they kick in they hit hard.
Edit: I gave the example with cows a human variant is Creutzfeldt–Jakob disease and onset of symptoms can happen years to decades after exposure. Average life expectancy following the onset of symptoms is 13 months. [1]
[0] https://en.wikipedia.org/wiki/Bovine_spongiform_encephalopat...
[1] https://en.wikipedia.org/wiki/Variant_Creutzfeldt%E2%80%93Ja...