'There’s good news and bad news. Vaccines’ efficacy did seem to decline – whether that was due to the appearance of the Delta variant or for other reasons. A double-vaxxed person, who had 92 per cent protection against severe Covid no matter which vaccine they received, saw that protection decrease steadily in approximately the first ten weeks (nearly two-and-a-half months) after the second dose. The good news is that the mRNA vaccines – Pfizer and Moderna – only declined slightly, and seemed to stabilise after that point: they levelled out at about 90 per cent efficacy against severe disease after 20 weeks. Alas, in this study at least, the AstraZeneca vaccine kept reducing in its efficacy: by 20 weeks, its protection against severe disease was as low as 60 per cent.
What about the longer term? Does AstraZeneca just wane and wane to absolute zero? Or does its efficacy drop for a while and then level out like the mRNA vaccines? The authors did some extrapolation: they produced models of what their data predicted would happen in future. This is educated guesswork, but nevertheless: one of their models implied AstraZeneca would keep declining to the point that, by 30 weeks (almost seven months), people who got it would have essentially no difference in risk from someone who hadn’t been vaccinated at all. This seems unlikely to be true, but the fact that the data even points in this direction should make us concerned about the AstraZeneca shot in the long term.'
From the above article. This study was in relation to severe covid resulting in hospitalisation. The data showed at 20 weeks az had declined to 60%. Modelling then looked forward to 30 weeks taking it down to 0 protection. It does say this is unlikely but we will only see I guess once we have passed this length of time.