There are multiple studies that point to VE declining over time, to about 50% after 6 months. No further data is available. I do not understand why this vaccine behavior was not observed and documented by the original clinical trials, which were conducted over a period of 4 months (July-Nov 2020). The Pfizer FDA EUA application claimed a blanket 95% vaccine efficacy.
https://www.fda.gov/media/144416/download
(OA) US, veterans, 3 months VEaI = 78%, 6 months VEaI = 50%, https://www.science.org/doi/10.1126/science.abm0620
US, 5 months VEaI = 47%, https://www.thelancet.com/journals/lancet/article/PIIS0140-6...
UK, 3 months VEaI = 75%. https://www.nature.com/articles/s41591-021-01548-7
Qatar, 3 months VEaI = 70%, 6 months VEaI = 12.8%, https://www.nejm.org/doi/full/10.1056/NEJMoa2114114
(preprint) Sweden, <50yo, 2-4 months VEaI = 83%, 4-6 months VEaI = 50%, https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3949410