First, vaccines aren’t 100% effective. Not everyone who is inoculated will respond in the same way. Those who are elderly or whose immune systems are faulty, damaged or stressed by some other illness are less likely to mount a robust response than someone younger and fitter. Covid-19 vaccines are highly effective but some people will still be vulnerable to the virus even after receiving their shots.
Second, the risk of dying from Covid-19 increases steeply with age. If a vaccine reduces an 80-year-old’s risk of death from Covid-19 by 95%, for instance, that 80-year-old’s risk of death might still be greater than the risk faced by an unvaccinated 20-year-old. Some chronic illnesses such as diabetes, hypertension and lung disease are also associated with a higher risk of severe illness and death.
Third, as more of the population gets vaccinated, there are fewer unvaccinated people for the virus to infect. If the pool of vaccinated people is larger than the pool of unvaccinated people, then it is possible and even likely that breakthrough infections resulting in death in the older, vaccinated group would match or exceed deaths in the younger, unvaccinated group. Consider an imaginary country with 100% of people vaccinated, where the virus can still somehow spread. All Covid-19 deaths would be in vaccinated individuals.