The high rate of asymptomatic COVID-19 infections aboard the cruise ship Diamond Princess “could partially explain the high attack rate among” the passengers and crew, according to the Centers for Disease Control and Prevention.
Testing of the 3,711 passengers and crew aboard the ship – the source of the largest outbreak outside of China during the initial stages of the pandemic – revealed that 19.2% were positive for COVID-19, Leah F. Moriarty, MPH, and associates reported in the Morbidity and Mortality Weekly Report.
At the time of testing, 46.5% (331) of the 712 infected individuals were asymptomatic, and “statistical models of the Diamond Princess outbreak suggest that 17.9% of infected persons never developed symptoms,” wrote Ms. Moriarty of the CDC COVID-19 response team, and associates.
RNA from the SARS-CoV-2 virus was found on surfaces in cabins up to 17 days after they had been vacated by passengers but before the cabins had been disinfected, the investigators noted.
The Diamond Princess departed from Yokohama, Japan, on Jan. 20, 2020, and returned on Feb. 3 after making six stops in three countries. The ship was quarantined upon its return because a symptomatic passenger who had departed Jan. 25 in Hong Kong tested positive for SARS-CoV-2, Ms. Moriarty and associates explained.
Of the 381 people from the ship who were symptomatic and tested positive, 37 (9.7%) needed intensive care and 9 (1.3%) died. There were 428 Americans on the ship, of whom 107 (25.0%) tested positive and 11 remained hospitalized in Japan as of March 13, they said.
“Many other cruise ships have since been implicated in SARS-CoV-2 transmission,” the investigators said, including the Grand Princess, which sailed out of San Francisco with 3,571 people on Feb. 21 and returned to Oakland on March 8.
That ship had been the site of virus transmission during its previous voyage from Feb. 11 to Feb. 21, from which more than 20 cases have been identified. During the latter trip, 21 of 45 passengers and crew tested positive before the ship docked. During the subsequent land-based quarantine, there have been 78 positive tests among the 469 people tested as of March 21, a rate of 16.6%, the research team reported.
“Public health responses to cruise ship outbreaks require extensive resources,” they wrote. “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.”
SOURCE: Moriarty LF et al. MMWR. 2020 Mar 23;69[early release]:1-6.