The Surprising Role of CRUISE Ships in the Coronavirus Outbreak

What Lessons Can We Learn About COVID-19 from Outbreaks on Cruise Ships?

Enclosed environments can help the virus spread, but they can also be used to conduct research on the novel coronavirus.

Diamond Princess was a cruise ship on which COVID-19, the disease caused by the novel coronavirus was detected, providing a rare opportunity to understand characteristics of the novel coronavirus that are difficult to study in large populations. Some of the initial studies of the ship, where around 700 people were infected, helped us understand how easily the virus spreads, estimate the severity of the disease, and see that people can have the infection without showing any symptoms.

Information from research conducted in this way is very important for decision-makers dealing with the epidemic. John Ioannidis of Stanford University explains:

“Cruise ships are like an ideal experiment for an isolated community. You know who is there, who is at risk, and that you can take measures covering everyone. A study observing how the virus spreads in this setting will be very different from studies conducted in large populations, because those generally observe and test patients with severe symptoms.”

Diamond Princess

On February 1, a passenger who had left the Diamond Princess days earlier was diagnosed with COVID-19 in Hong Kong. Two days later, on February 3, the ship was quarantined in Japanese waters with its 3711 passengers and crew. By the following month, more than 700 people had been infected, and for weeks the ship had the largest number of infected cases outside China.

Along with the Diamond Princess, COVID-19 cases were reported on at least 25 other ships. These ships can be at risk during outbreaks because they are enclosed environments and host a high proportion of older people. One of the ships, with 78 cases, was the Grand Princess , which was also quarantined off the California coast.

Japanese authorities performed more than 3000 tests on the Diamond Princess, starting with older people and those showing symptoms. Some passengers were tested more than once to determine the speed and pattern of the virus's spread. Testing nearly all passengers and crew enabled researchers to obtain very useful information for tackling the outbreak. In large populations, people with no symptoms or only mild symptoms are usually not tested.

A Eurosurveillance team working with Diamond Princess data found that, among all passengers infected by February 20, 18% showed no symptoms. Co-author Gerardo Chowell, a mathematical epidemiologist at Georgia State University in Atlanta, says, “This is an important number”. However, the passengers included many older people who would be likely to develop severe disease if infected, so the proportion of people without symptoms is likely to be higher in the general population.

Disease Severity

Another team using the ship's data found that, among detected cases, the mortality rate was approximately 1.1%. This was much lower than the World Health Organization's (WHO) estimate of 3.8% based on China. The WHO calculates its estimate by dividing the total number of deaths by the number of detected cases. However, Timothy Russell, a mathematical epidemiologist at the London School of Hygiene and Tropical Medicine, believes that estimates made in settings where not everyone is screened turn out much higher than the true rate.

In contrast, Russell and his team published a study using data from the ship and from 72 thousand confirmed cases in China, and are currently awaiting peer review. The group also estimates the COVID-19 infection fatality rate in China to be approximately 0.5%. Calculating this rate in large populations is very difficult, because some deaths are not identified when people show no symptoms or are not tested.

Marc Lipsitch of the Harvard T.H. Chan School of Public Health believes that determining the infection fatality rate is important for public health workers, but that testing has a substantial effect on these estimates, because people who were infected but recovered may have been missed.

Ioannidis added that studies using the Diamond Princess data could benefit from information such as whether people had smoked, as part of the medical histories of those on board. “We know that not just age, but also the presence of medical conditions such as chronic obstructive pulmonary disease, heart disease, diabetes and other conditions increases the risk of a poor outcome”, he says.

Cabin Quarantine

Chowell also observed how the strict measures applied on the ship helped slow the spread of the virus. From February 5, passengers were isolated in their cabins for at least two weeks.

He and Kenki Mizumoto of Kyoto University published a report showing that, on the day quarantine began, one person could spread the virus to at least 7 people. They believe this rate was high because passengers lived in confined spaces and had many shared facilities.

After quarantine was introduced, the transmission rate fell to 1 person. This showed that the strict measures significantly reduced the virus's spread. It was still not perfect, because people could infect their cabinmates.

Ioannidis believes that the data collected from ships are very important, but that it is difficult to draw lessons for entire countries from the quarantine measures applied on them, adding: "An entire country is not a ship."

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