Thanks for the link. That's a very interesting study with some useful references. One of the graphs (Figure 2A) shows that after 4 months following a 2nd Pfizer dose, efficacy against Delta declined to 51.7% percent. The study I cited from the Lancet had efficacy after 4 months at 53%, so they're both pretty similar for the 4 month time period. It's the 20% vs 47% efficacy against infection after 6 months which is the big disparity. Both studies found, however, the protection against severe covid 6 months after full vaccination remained at 90%, so that's reassuring.
The Qatar study also said this:
"PCR testing in Qatar is done on a mass scale, with approximately 5% of the population being tested every week.5 Approximately 75% of those who receive a diagnosis of SARS-CoV-2 infection at present do so not because of the appearance of symptoms but because of routine testing. It is possible that many asymptomatic infections were diagnosed among vaccinated participants that otherwise would have been missed. The higher ascertainment of infection may have lowered the effectiveness estimates."
Also, the Qatar study went through August whereas the Kaiser SoCal study went through July, so there was a longer period with significant Delta cases in Qatar vs. SoCal.
