I think a big problem with these studies is that someone who gets a Shingles or RSV vaccine is not, on average, like a person who does not.
Either, the person who is vaccinated is well enough organised, capable of deciding to do something to maintain their health, making the appointment and getting themselves there. They probably regularly visit the doctor for their other health needs. In which case, they are still thinking well and are regularly seen by a doctor, and so are, at worst, some years off getting diagnosed with dementia.
And/or, the person who is vaccinated lives in a situation where someone is looking after their health needs and getting vaccinations done, and so they are also being closely monitored for signs of dementia. So again, if they had dementia, they would probably have already been diagnosed with it. I imagine in a retirement home, there is more funding for a resident with dementia, so it would be in the retirement home's interests to get the diagnosis fairly quickly.
Contrast that with someone who doesn't get the vaccine. Someone in the first stages of dementia and living on their own will be struggling to hold everything together, and probably won't be thinking about discretionary activities such as getting a vaccine. If the unvaccinated person is living with a spouse or a child who isn't able to arrange for the vaccine or doesn't believe in vaccines, then they probably aren't getting to the doctor often either, and may not be being closely monitored for signs of dementia.
If a person who is vaccinated has any signs of dementia, they probably will have already been diagnosed with it already. So, for them, the chance of being diagnosed in those next 18 months is only the chance of a new onset that moves very swiftly.
Whereas, a person who is not vaccinated may already have quite significant signs of dementia but be undiagnosed. So, for them, the chance of being diagnosed in those next 18 months is the chance of an onset in quite a lot of years prior as well as the next 18 months.
