This is when the second vaccination is given and that response requires another week before it is maximal. So it is when vaccinated subjects have reached this point that the protection is highest. For a single-dose vaccine like the Johnson and Johnson vaccine, it also takes a few weeks before maximal protection is achieved.
Therefore, it is totally understandable that someone could get vaccinated and then be exposed to the coronavirus before the level of immune protection is great enough to halt the infection.
This is why it is extremely important that you follow all social distancing guidelines and mask-wearing until at least a week after completing both vaccinations or 4 weeks after a single dose vaccination!
Another possibility is that a person could already be infected before they get their vaccination but just not experiencing symptoms.
This can occur because no type of COVID-19 tests are required prior to getting the vaccine.
It should be noted, at least in clinical studies, people who got infected after being vaccinated generally did not require hospitalization, and none of them died. So the vaccine still provided an important benefit.
The question of how well these vaccines will protect us against the new mutant strains of the virus remains to be answered but scientists are working extremely hard to answer this critical question.
Stay tuned; new information is continuously coming in.