Preface: I have been in favor of the COVID vaccine and disease mitigations (and wish we would have used this opportunity for clean indoor air...).

I'm willing to accept my memory is wrong here with evidence, but I remember a very strong narrative in the early period claiming that the vaccine did in fact prevent contraction and transmission, to the point where it was supposedly surprising when "breakthrough" cases started being reported.

It's possible there was some loose language around "prevent" as I did see that especially later on, but I have trouble finding reliable information on what they actually believed and if they actually reported this accurately to the public.

There is the unfortunate mark against where they knowingly promoted misinformation around masks - persistent through today - that they were ineffective, in an effort to direct uncontrolled distribution of masks to medical professionals most in need.