From the phase 3 trials you'd absolutely expect cases to occur after vaccinations. That's what the 95% efficacy after 2 doses means, that 5% of people had symptomatic infections.

Hopefully it is the case that the 5% are people that have a weak response to the vaccine and the remainder mostly aren't infectious. They can study this by monitoring for asymptomatic infections (I don't know if they are going to or not).