When COVID-19 was detected among passengers on the cruise ship Diamond Princess, the vessel offered a rare opportunity to understand features of the new coronavirus that are hard to investigate in the wider population. Some of the first studies from the ship - where some 700 people were infected - have revealed how easily the virus spreads, provided estimates of the disease's severity and allowed researchers to investigate the share of infections with no symptoms.
Information gleaned from such outbreaks is crucial for people making decisions on how to manage the epidemic, say researchers.
"Cruise ships are like an ideal experiment of a closed population. You know exactly who is there and at risk and you can measure everyone," says John Ioannidis, an epidemiologist at Stanford University in California. This is very different from trying to study the spread in a wider population, where only some people, typically those with severe symptoms, are tested and monitored.
On 1 February, a passenger who had disembarked from the Diamond Princess days earlier in Hong Kong tested positive for the COVID-19 coronavirus. The ship was quarantined immediately after it arrived in Japanese waters on 3 February, with 3,711 passengers and crew members on board. Over the next month, more than 700 people on board were infected - including a nurse - and for weeks the ship was the site of the largest outbreak outside China.
Outbreaks seed easily on the vessels because of the close confines and high proportions of older people who tend to be more vulnerable to the disease. Since the Diamond Princess, at least 25 other cruise ships have confirmed COVID-19 cases - including 78 cases on the Grand Princess, which was quarantined off the coast of California. Returned passengers have also seeded outbreaks in countries including the United States.
Japanese officials performed more than 3,000 tests on the Diamond Princess, starting with older passengers and those with symptoms. Some passengers were tested more than once, offering insight into the virus's spread over time. Testing almost all of the passengers and crew helped researchers to understand a key blind spot in many infectious-disease outbreaks - how many people are actually infected, including those who have mild symptoms or none at all. These cases often go undetected in the general population.
Using the Diamond Princess data, a team reports in Eurosurveillance that by 20 February, 18% of all infected people on the ship had no symptoms. "That is a substantial number," says co-author Gerardo Chowell, a mathematical epidemiologist at Georgia State University in Atlanta. But the passengers included a large number of elderly people, who are most likely to develop severe disease if infected, so the share of asymptomatic people in the general population is likely to be higher, he says.
Another team used data from the ship to estimate that the proportion of deaths among confirmed cases in China, the case fatality rate (CFR), was around 1.1% - much lower than the 3.8% estimated by the World Health Organization (WHO).
The WHO simply divided China's total number of deaths by the total number of confirmed infections, says Timothy Russell, a mathematical epidemiologist at the London School of Hygiene and Tropical Medicine. That method does not take into account that only a fraction of infected people are actually tested, and so it makes the disease seem more deadly than it is, he says.
By contrast, Russell and his colleagues used data from the ship - where almost everyone was tested, and all seven deaths recorded - and combined it with more than 72,000 confirmed cases in China, making their CFR estimate more robust. The results have been posted on the biomedical preprint server medRxiv, and have not been peer-reviewed yet.
The group also estimates that the infection fatality rate (IFR) in China - the proportion of all infections, including asymptomatic ones, that result in death - is even lower, at roughly 0.5%. The IFR is especially tricky to calculate in the population, because some deaths go undetected if the person didn't show symptoms or get tested.
The IFR is an important indicator to help public-health officials understand disease severity and how to intervene, says Marc Lipsitch, an infectious-disease epidemiologist at the Harvard T.H. Chan School of Public Health in Boston, Massachusetts. "This is an important effort, but one important caveat is that the infections were ascertained by viral testing" and might have missed people who had been infected but recovered, he says.
Ioannidis adds that the studies using Diamond Princess data could benefit from the addition of the medical histories of those on board, such as whether or not people smoked. "We know that not only age, but also presence of medical diseases such as chronic obstructive pulmonary disease, heart disease, diabetes and other conditions increase the risk of a bad outcome," he says.
Chowell also looked at how effective the stringent containment measures introduced on the Diamond Princess were in reducing the virus's spread. From 5 February, passengers on the ship were confined to their cabins for two weeks or more.
He and Kenji Mizumoto, an epidemiologist at Kyoto University in Japan, report in Infectious Disease Modelling that the day the quarantine was introduced, one person could go on to infect more than 7 others. The infection rate was probably quite high because people were living in close quarters and touching surfaces contaminated with the virus, says Chowell.
But after people were confined to their rooms, the average number of others to whom one infected person passed the virus dropped below one. This suggests that the quarantine averted a lot of infections, says Chowell. However, it wasn't perfect: passengers could still infect their room-mates and crew members, he says.
Although insights from the ship about the virus's spread and severity are valuable, it is difficult to draw lessons from its quarantine for those countries implementing similar lockdown measures, says Ioannidis. "A whole country is not a ship."
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