It turns out that it really was quite a good place to study COVID-19. Thanks to a post on Watts Up With That and hat tip to Borepatch for seeing it first, we get to learn some pretty remarkable facts about COVID-19.
To begin with, it really was a scenario almost designed to provide massive numbers of infected people and mass casualties; a way to observe the virus spread across the thousands on board, perhaps killing hundreds. If the fatality rate was the kind of number people were talking about months ago (10% or more), that could have meant 400 killed.
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.Patient zero was on board on January 20th to the 25th. They man wasn't diagnosed until February 1st. In those five days, he spread the virus. In the next week, the situation was like that previous paragraph. People unaware they were spewing virus working in close contact with each other.
- It was a "worst case" scenario, as people were confined for weeks with other infected people in very close quarters. While this protected the general public from the infection, it likely maximized the spread of the virus on board the ship.
- It was a "worst case" scenario as the population on board skewed dramatically to older - and thus, more vulnerable - people.
- 83% (confidence interval of 82.7% – 83.9%) of the passengers never got the disease at all. Said the other way, only 17% of the passengers were infected.
- The oldest portion of the passengers, over 80 years old, had a higher infection rate: 25%, but that's not even twice the rate of the general population.
- Wait - it gets better. Slightly less than half the passengers (48.6% ± 2.0%) who would test positive for the disease did NOT get sick and showed NO symptoms. They never knew they had it.
- The young (under 20) and old (over 50) disproportionally showed no symptoms after being infected
- The overall, age-adjusted death rate was 1.2% (7 cases total)
This shows the percentage of each age group on board the Diamond Princess who did NOT contract the virus. The youngest, birth to 9 years, looks to say 93% did not test positive for the virus. Note that the lowest percentage who didn't test positive is the 80-89 year old cohort, which says the chance of contracting the virus seem so go up with age. Note how the colored bars are shorter for ages 50-59 than the three groups above it, and then shorter every additional decade.
The next plot is percentage of each age group who showed no symptoms.
The birth to 9 years old group was 100% symptom free. From the first plot, roughly 7% of that age group tested positive and that test was the only way it could be known they had the virus. We can keep going on those; the second group in first plot shows 78% did not contract the virus, so 22% did. Of the 22%, 60% showed no symptoms. Even in the oldest group, with 25% testing positive for the virus, almost half of them (45%) had no symptoms.
The study itself is Estimating the infection and case fatality ratio for COVID-19 using age-adjusted data from the outbreak on the Diamond Princess cruise ship (PDF warning). The study authors define an age-adjusted Infection Fatality Rate of 1.2% with a possible interval of 0.38%–2.7%. The wide uncertainty range is due to the small number of deaths. Although this might not be relevant to an epidemiologist studying the disease, I see this as 7 deaths out of 3,711 passengers and crew on the Diamond Princess. I don't see any reason to exclude the 83% who didn't get infected because they were exposed and could have been infected. 7 deaths out of 3711 people on board is a 0.19% fatality rate. I hear the number 0.1% for the annual flu, but I don't know if that compares more directly to my raw numbers or their age-adjusted 1.2%.
These 3711 people were in a virtual incubator for the virus and there's every reason to think they were all at risk. According to this article, a Japanese Infectious Diseases expert went onboard the Diamond Princess and was afraid of how poorly they implemented their quarantine. The fact that 83% never got the disease hints that it may not be as communicable as everyone fears.
It seems that the more I learn about COVID-19, the less concerned I am about it.