[spuz]

If you look at the formula for determining vaccine efficacy, you'll see it's determined by comparing the risk of being infected while unvaccinated vs the risk of being infected while vaccinated. If the risk of being infected while vaccinated is higher than the risk while unvaccinated then you will get a negative efficacy.

I believe you are right in saying we simply don't know what the efficacy is for those over 75 as a result of this study. Given that the range extends to 100% that tells us that no one over 75 who received the vaccine also tested positive for the virus. However the lower bound tells us that even if the vaccine had increased the chance of being infected by 13% we still would have expected 0 over 75 to be infected by the time they were tested simply because the probability of being infected is already so low.

https://en.m.wikipedia.org/wiki/Vaccine_efficacy

[pepperberg]

Thank you for your explanation!