Cruise ships are often settings for outbreaks of infectious diseases, due to their closed environment and contact between travellers from many countries, a new report said.
According to the report on COVID-19 affecting cruise ships in February and March of this year, the US Centres for Disease Control and Prevention (CDC) claimed that outbreaks on cruise ships posed a risk for rapid spread of disease beyond the voyage.
Aggressive efforts are required to contain the spread. All persons should defer all cruise travel worldwide during the COVID-19 pandemic, the CDC warned.
During February/March, 2020, COVID-19 outbreaks associated with three cruise ship voyages caused more than 800 laboratory-confirmed cases among passengers and crew, including 10 deaths. Transmission occurred across multiple voyages of several ships.
The report described public health responses to COVID-19 outbreaks on these ships. During 7th to 23rd February, 2020, the largest cluster of COVID-19 cases outside mainland China occurred on the ‘Diamond Princess’, which was quarantined in the port of Yokohama, Japan, on 3rd February.
On 6th March, cases of COVID-19 were identified on the ‘Grand Princess’ off the Californian coast. She was subsequently quarantined. By 17th March, confirmed cases of COVID-19 had been associated with at least 25 additional cruise ship voyages.
As a result, on 21st February, CDC recommended avoiding travel on cruise ships in Southeast Asia; on 8th March, this recommendation was broadened to include deferring all cruise ship travel worldwide for those with underlying health conditions and for persons aged ≥65 years.
On 13th March, the Cruise Lines International Association (CLIA) announced a 30-day voluntary suspension of cruise operations in the US. CDC issued a Level 3 travel warning on 17th March, recommending that all cruise travel be deferred worldwide.
The ‘Diamond Princess’ and ‘Grand Princess’ had more than 800 total COVID-19 cases, including 10 deaths. In the US, between 3rd February and 13th March, around 200 cases of COVID-19 were confirmed among returned cruise travellers from multiple ship voyages, including the two Princess ships, accounting for about 17% of total reported US cases at the time.
Cases linked with cruise travel have been reported to CDC in at least 15 US states. Since February, multiple international cruises have been implicated in reports of COVID-19 cases, including at least 60 cases in the US from Nile River cruises in Egypt. Secondary community-acquired cases linked to returned passengers on cruises have also been reported, the report said.
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 travellers and responders.
These responses required the co-ordination of stakeholders across multiple sectors, including US 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, (eg, 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 US returned travellers.
Cruise ships are often settings for outbreaks of infectious diseases because of their closed environment, contact between travellers 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.
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 suggested that 17.9% of infected persons never developed symptoms. 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 this data cannot be used to determine whether transmission occurred from contaminated surfaces, further study of fomite transmission of SARS-CoV-2 on board cruise ships is warranted, CDC said.
Factors that facilitate spread on cruise ships might include mingling of travellers 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. 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, the report said.
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 use of travel notices and warnings for conveyances to limit disease transmission.