Imagine if there is an infectious disease that spreads within cities but not across cities. Throughout the COVID crisis, the city specific infection rate has varied across cities at each point in time. In a city facing a rising infection rate, people can adapt by either engaging in costly self protection (self isolating) or through public health interventions --- such as vaccinating the local population. In this case, public health substitutes for private health protection investments. To be safe, one can either isolate and not be vaccinated or continue with life as usual and be vaccinated.