With so many staff on a cruise ship, can the health of the crew be guaranteed?

Staff on regular cruise ships are required to have regular medical checkups, so their health is still guaranteed. However, such considerations are still possible, for example, during the worst of the epidemic last year, cruise ships docked in Japan, there was a serious centralized infection phenomenon, mainly because of the cruise ship's internal circulation of air conditioning, in this relatively airtight environment, will be infected with viruses will be further spread to the entire cruise ship environment, resulting in serious infection of the entire cruise ship, which is a serious health risk for both staff and tourists. This is a serious health hazard for both staff and tourists. This is a serious health risk for both staff and tourists. Therefore, in addition to regular health checks, cruise ship staff must be vaccinated against common infectious diseases in order to prevent the risk of infection in this confined environment.