Diamond Princess Mysteries

OK, here are my questions. We had a perfect petri-dish coronavirus disease (COVID-19) experiment with the cruise ship “Diamond Princess”. That’s the cruise ship that ended up in quarantine for a number of weeks after a number of people tested positive for the coronavirus. 

I got to wondering what the outcome of the experiment was. So I dug around and found an analysis of the situation, with the catchy title of Estimating the infection and case fatality ratio for COVID-19 using age-adjusted data from the outbreak on the Diamond Princess cruise ship (PDF), so I could see what the outcomes were. 

As you might imagine, before they knew it was a problem, the epidemic raged on the ship, with infected crew members cooking and cleaning for the guests, people all eating together, close living quarters, lots of social interaction, and a generally older population.  

Seems like a perfect situation for an overwhelming majority of the passengers to become infected. And despite that, some 83% (82.7% – 83.9%) of the passengers never got the disease at all … why? 

Let me start by looking at the age distribution of the Diamond Princess, along with the equivalent age distribution for the entire US.

 

Figure 1. Number of passengers by age group on the Diamond Princess (solid) and expected number of passengers given current US population percentages (hatched).

When as a young man I lived in a port town with cruise ships calling, we used to describe the passengers as “newlyweds and nearlydeads”. Hmmm … through some improbable series of misunderstandings and coincidences, I’m in the orange zone now … but I digress … In any case, Figure 1 shows the preponderance of … mmm … I’ll call them “folks of a certain distinguished age” on the Diamond Princess. Folks you’d expect to be hit by diseases. Next, here’s the breakdown of how many people didn’t get the virus, by age group:

Figure 2. Percentage of unaffected passengers on the Diamond Princess. “Whiskers” on the plot show the uncertainty of each percentage.

In addition to the low rate of disease incidence (83% didn’t get it), the curious part of Figure 2 for me is that there’s not a whole lot of difference between young and old passengers in terms of how many didn’t get coronavirus. For example, sixty to sixty-nine-year-old passengers stayed healthier than teenagers. And three-quarters of the oldest group, those over eighty, didn’t get the virus. Go figure.

GO READ THE WHOLE THING.