Coronavirus survived in cabins on Diamond Princess for up to 17 days after passengers disembarked in Japan last month.
The Princess Cruises vessel was carrying 3,711 passengers and crew when it had to be quarantined in the port of Yokohama after Covid-19 had been detected.
A report by the US Centers for Disease Control (CDC) highlights how long the virus remained on surfaces in the cabins – after the passengers left but before disinfection procedures.
The CDC said 712 of the passengers and crew tested positive and almost half (46.5%) were asymptomatic at the time of testing.
Among 381 symptomatic patients, 37 required intensive care, and nine died, including the first Briton to die in the pandemic.
The report said: “A high proportion of asymptomatic infections could partially explain the high attack rate among cruise ship passengers and crew.”
It added: “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.”
The report also looked at the outbreak on sister ship the Grand Princess, which sailed on a round-trip from California, making four stops in Mexico in February.
Most of the 1,111 crew and 68 passengers from that voyage remained on board for a second voyage that departed from San Francisco on February 21.
Passengers were confined to their cabins following the death of a guest from coronavirus after the previous cruise.
The CDC said more than 800 cases of Covid-19 cases occurred during outbreaks on Grand Princess and Diamond Princess, and there have been 10 deaths.
In a statement issued by Princess Cruises, the line stressed the findings did not prove the virus that causes COVID-19 was transmitted via the cabin surfaces.
“According to epidemiologists with Japan’s National Institute of Infectious Diseases, what was detected on surfaces was SARS-CoV-2 RNA*, not live virus, in select cruise ship cabins after they were vacated, and this testing was intentionally conducted before disinfection occurred,” Princess said.
“These findings were expected because Princess Cruises voluntarily preserved these staterooms for this testing.”
RNA is short for ribonucleic acid – a genetic material which carries the genetic information of many viruses. It can indicate if the virus was present but does not indicate the virus was still alive.
“The findings do not indicate that transmission of the virus that causes COVID-19, was transmitted via these surfaces. More research must be completed to determine if virus can be transmitted by SARS-CoV-2 fomites (objects or materials which are likely to carry infection, such as clothes, utensils, and furniture).”