When one person claims the efficacy has dropped from, say, 90% to 80% in six months, and the next person claims that person is "wrong" because "reasons", and says its 88% to 69% in five months, the probability is that they are both wrong, or they could be both right, or even that the comparison is meaningless in any specific way. Why is this?
Its because a set of numbers like vaccine efficacy being reduced from X% to Y% in N months does not apply universally everywhere. Its really important to understand that the vaccine, the virus itself as it mutates, and the environment in which the infections are taking place are all variables in a very complex calculation, the results of which impact on the R
0 of the disease, for that environment. It is pretty much meaningless to compare studies done by different people, using different subjects, and different vaccines in different environments.
The environment in which the transmissions is taking place is a very, very important part of the equation. In an environment of lockdown, there are less opportunities for the virus to spread, so there will be less infections and therefore less breakthrough cases, and the vaccine will appear to be more effective. Conversely, environments where you have a more mobile population with closer social contact among individuals will have higher transmission rates, therefore higher chances of breakthrough cases, and therefore the vaccination will look less effective.
Also, the transmissibility of the disease is influenced by the environment. In New Zealand the Delta variant is running at an R
0 of about 1 (meaning one person infects one other) because we are in full lockdown and are managing to flatten the curve. In Sydney, the R
0 is about 1.3, but in the UK and the US, they are running at about 3 to 4, and in other countries that are not running lockdowns at all it is as high as 6 to 7. Environment is key in getting on top of the situation.
Finally, the really, really important thing keep in mind is that the efficacy in
prevention against infection might be reducing over time, but the protection against becoming seriously ill does not appear to be. There is as yet no evidence that when vaccinated people get breakthrough infections, they become more seriously ill the longer they have been vaccinated.
Sources.
https://www.timeout.com/sydney/news...to-follow-in-australias-delta-outbreak-081821
https://www.fau.edu/newsdesk/articles/delta-vaccines-faqs.php
https://www.nature.com/articles/d41586-021-02261-8
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6466347/
https://www.forbes.com/sites/leahro...ng-against-the-delta-variant/?sh=2b24dfa71a6f