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...
Very few long-term diseases are long-term because of a deterministic countdown. Rather, they’re the result of a small but long-lived cumulative probability. The cancer that takes twenty years to show up is caused by the persistent 0.17% chance of a malignant tumor every month.
But the upshot is that if you start with a sample size of 100,000 people, the cancer risk will be glaringly obvious in the first month.