CDC: Asymptomatic Transmission Contributed to Severity of Cruise Ship Outbreak

Nearly half of the passengers and crew on the Diamond Princess cruise ship who tested positive for the new coronavirus did not have symptoms at the time of testing, according to a new report from CDC. The Diamond Princess was the setting of the largest COVID-19 outbreak outside China in the early stages of the pandemic.

Carrying approximately 3,700 passengers and crew, the Diamond Princess departed Yokohama, Japan, on January 20 and made six stops in three countries before returning to port. The ship was quarantined February 3 after Japanese authorities were notified that a passenger who disembarked in Hong Kong had tested positive for the virus that causes COVID-19. Passengers were quarantined in their cabins on February 5, but crew continued to work.

The CDC report indicates that disease transmission occurred largely before quarantine for passengers and afterward for crew. Statistical models suggest that 17.9 percent of those infected never developed symptoms. At the time of testing, 46.5 percent of those infected were asymptomatic. According to CDC, the high percentage of asymptomatic infections may help explain why so many people on board were infected.

Passengers were released from the ship in phases. More than 1,000 were repatriated to their home countries in mid-February. The last of the passengers disembarked February 24. Some crew remained on board until March 1.

RNA of the virus that causes COVID-19 was found on surfaces within the ship up to 17 days after cabins were evacuated but prior to disinfection.