Respiratory virus vaccines can suffer this issue because of how quickly those kinds of viruses mutate. Flu vaccines are notorious for not reducing the mortalit

Respiratory virus vaccines can suffer this issue because of how quickly those kinds of viruses mutate. Flu vaccines are notorious for not reducing the mortality of the elderly whilst being reported as highly effective. They can even increase mortality from the flu due to late response caused by overfitting. The problem is twofold here: the vaccines cause displacement so they look good in trials, but then they also look good in observational data because the sort of people who sign up voluntarily for flu shots are just paying much more attention to their health in general so there's a selection bias effect. This paper tried to fix the selection bias and: https://pmc.ncbi.nlm.nih.gov/articles/PMC2728831/ https://pmc.ncbi.nlm.nih.gov/articles/PMC2728831/ "We found that flu shots reduced all-cause mortality among elderly Kaiser Permanente members by 4.6% during 9 laboratory-defined flu seasons in Northern California. Other researchers have reported that flu shots reduce mortality by much greater amounts." Overfitting can exacerbate the problem by causing displacement to a variant that's worse. Here's an example from 2009 where Canadian authorities investigated the effectiveness of the trivalent influenza vaccine on swine flu: https://academic.oup.com/cid/article/51/9/1017/292207?login=false https://academic.oup.com/cid/article/51/9/1017/292207?login=... "An outbreak investigation in British Columbia during the late spring of 2009 provided the first indication of an unexpected association between receipt of TIV and pH1N1 illness. This led to 5 additional studies through the summer 2009 in Canada, each of which corroborated these initial findings." The reason there's so much controversy around vaccines in general is that the techniques used can easily look as if they're working well whilst they actually don't. Mistakes are understandable, but the incentive is then to cover things up because public health officials and researchers are terrified of doing anything that might support "anti vaxx" narratives (e.g. admitting to mistakes).