One word to keep it simple for you. Delta.
It can't get any simpler.
On Feb. 11, the World Health Organization indicated the AZD1222 vaccine efficacy of 63.09% against the development of symptomatic SARS-CoV-2 infection. The conclusions of the Chau paper support the warnings by leading medical experts that the partial, non-sterilizing immunity from the three notoriously “leaky” COVID-19 vaccines allow carriage of 251 times the viral load of SARS-CoV-2 as compared to samples from the pre-vaccination era in 2020.
Why is this so hard for you?
I bet if you took off your Fauci kneepads you could figure it out.