This is where the Diamond Princess data provides important insight. Of the 3711 people on board, at least 705 have tested positive for the virus (which, considering the confines, conditions, and how contagious this virus appears to be, is surprisingly low). Of those, more than half are asymptomatic, while very few asymptomatic people were detected in China. This alone suggests a halving of the virus’s true fatality rate.
On the Diamond Princess, six deaths have occurred among the passengers, constituting a case fatality rate of 0.85 percent. Unlike the data from China and elsewhere, where sorting out why a patient died is extremely difficult, we can assume that these are excess fatalities—they wouldn’t have occurred but for SARS-CoV-2. The most important insight is that all six fatalities occurred in patients who are more than 70 years old. Not a single Diamond Princess patient under age 70 has died. If the numbers from reports out of China had held, the expected number of deaths in those under 70 should have been around 4.
The data from the Diamond Princess suggest an 8-fold lower mortality amongst patients older than 70 and 3-fold lower mortality in patients over 80 compared to what was reported in China initially. But even those numbers, 1.1 percent and 4.9 percent respectively, are concerning. But there’s another thing that’s worth remembering: These patients were likely exposed repeatedly to concentrated viral loads (which can cause worse illness).