Most of what I've seen about the AZ vaccine's efficacy is that it ranks quite low. It's about as effective with two shots as J&J is with one shot. (Which partially explains the surge in the UK.) In fact, a number of countries that started off with AZ as the first dose (such as Canada, which delayed the second dose a ridiculously long time) are now giving an mRNA shot as the second dose instead of the second AZ shot. The WHO doesn't like that.
With as many vaccines as have been administered here in the USA, there really isn't a reason to introduce the AZ shot. It adds nothing to our arsenal except more doses.