What happens when you're late for your second Pfizer jab?

Another morning with Prof Tulio de Oliveria from UKZN's KRISP. He joins us answering all your COVID-19 and vaccine related question.
In today's questions he got asked the following:
- How is it possible that some countries have vaccinated enough people to reach population immunity but are still experiencing a fourth wave of infections and are now requiring 3rd booster shots. For example Israel, England? From KM
- Why are vaccine injections administered in the upper arm and not in the bum or thigh? Oliver Cash - name.
- How serious it it if you are late for your second Pfizer jab?