Discussion

Public health responses to COVID-19 outbreaks on cruise ships were aimed at limiting transmission among passengers and crew, preventing exportation of COVID-19 to other communities, and assuring the safety of travelers and responders. These responses required the coordination of stakeholders across multiple sectors, including U.S. Government departments and agencies, foreign ministries of health, foreign embassies, state and local public health departments, hospitals, laboratories, and cruise ship companies. At the time of the Diamond Princess outbreak, it became apparent that passengers disembarking from cruise ships could be a source of community transmission. Therefore, aggressive efforts to contain transmission on board and prevent further transmission upon disembarkation and repatriation were instituted. These efforts included travel restrictions applied to persons, movement restrictions applied to ships, infection prevention and control measures, (e.g., use of personal protective equipment for medical and cleaning staff), disinfection of the cabins of persons with suspected COVID-19, provision of communication materials, notification of state health departments, and investigation of contacts of cases identified among U.S. returned travelers.

Cruise ships are often settings for outbreaks of infectious diseases because of their closed environment, contact between travelers from many countries, and crew transfers between ships. On the Diamond Princess, transmission largely occurred among passengers before quarantine was implemented, whereas crew infections peaked after quarantine (6). On the Grand Princess, crew members were likely infected on voyage A and then transmitted SARS-CoV-2 to passengers on voyage B. The results of testing of passengers and crew on board the Diamond Princess demonstrated a high proportion (46.5%) of asymptomatic infections at the time of testing. Available statistical models of the Diamond Princess outbreak suggest that 17.9% of infected persons never developed symptoms (9). A high proportion of asymptomatic infections could partially explain the high attack rate among cruise ship passengers and crew. SARS-CoV-2 RNA was identified on a variety of surfaces in cabins of both symptomatic and asymptomatic infected passengers up to 17 days after cabins were vacated on the Diamond Princess but before disinfection procedures had been conducted (Takuya Yamagishi, National Institute of Infectious Diseases, personal communication, 2020). Although these data cannot be used to determine whether transmission occurred from contaminated surfaces, further study of fomite transmission of SARS-CoV-2 aboard cruise ships is warranted.

During the initial stages of the COVID-19 pandemic, the Diamond Princess was the setting of the largest outbreak outside mainland China. Many other cruise ships have since been implicated in SARS-CoV-2 transmission. Factors that facilitate spread on cruise ships might include mingling of travelers from multiple geographic regions and the closed nature of a cruise ship environment. This is particularly concerning for older passengers, who are at increased risk for serious complications of COVID-19 (4). The Grand Princess was an example of perpetuation of transmission from crew members across multiple consecutive voyages and the potential introduction of the virus to passengers and crew on other ships. Public health responses to cruise ship outbreaks require extensive resources. Temporary suspension of cruise ship travel during the current phase of the COVID-19 pandemic has been partially implemented by cruise lines through voluntary suspensions of operations, and by CDC through its unprecedented use of travel notices and warnings for conveyances to limit disease transmission (5).