There are many compelling reasons to conclude that SARS-CoV-2, the virus that causes COVID-19, is not nearly as deadly as is currently feared. But COVID-19 panic has set in nonetheless. You can’t find hand sanitizer in stores, and N95 face masks are being sold online for exorbitant prices, never mind that neither is the best way to protect against the virus (yes, just wash your hands). The public is behaving as if this epidemic is the next Spanish flu, which is frankly understandable given that initial reports have staked COVID-19 mortality at about 2–3 percent, quite similar to the 1918 pandemic that killed tens of millions of people.
Allow me to be the bearer of good news. These frightening numbers are unlikely to hold. The true case fatality rate, known as CFR, of this virus is likely to be far lower than current reports suggest. Even some lower estimates, such as the 1 percent death rate recently mentioned by the directors of the National Institutes of Health and the Centers for Disease Control and Prevention, likely substantially overstate the case.
We shouldn’t be surprised that the numbers are inflated. In past epidemics, initial CFRs were floridly exaggerated. For example, in the 2009 H1N1 pandemic some early estimates were 10 times greater than the eventual CFR, of 1.28 percent. Epidemiologists think and quibble in terms of numerators and denominators—which patients were included when fractional estimates were calculated, which weren’t, were those decisions valid—and the results change a lot as a result. We are already seeing this. In the early days of the crisis in Wuhan, China, the CFR was more than 4 percent. As the virus spread to other parts of Hubei, the number fell to 2 percent. As it spread through China, the reported CFR dropped further, to 0.2 to 0.4 percent. As testing begins to include more asymptomatic and mild cases, more realistic numbers are starting to surface. New reports from the World Health Organization that estimate the global death rate of COVID-19 to be 3.4 percent, higher than previously believed, is not cause for further panic. This number is subject to the same usual forces that we would normally expect to inaccurately embellish death rate statistics early in an epidemic. If anything, it underscores just how early we are in this.
But the most straightforward and compelling evidence that the true case fatality rate of SARS-CoV-2 is well under 1 percent comes not from statistical trends and methodological massage, but from data from the Diamond Princess cruise outbreak and subsequent quarantine off the coast of Japan.
A quarantined boat is an ideal—if unfortunate—natural laboratory to study a virus. Many variables normally impossible to control are controlled. We know that all but one patient boarded the boat without the virus. We know that the other passengers were healthy enough to travel. We know their whereabouts and exposures. While the numbers coming out of China are scary, we don’t know how many of those patients were already ill for other reasons. How many were already hospitalized for another life-threatening illness and then caught the virus? How many were completely healthy, caught the virus, and developed a critical illness? In the real world, we just don’t know.
Here’s the problem with looking at mortality numbers in a general setting: In China, 9 million people die per year, which comes out to 25,000 people every single day, or around 1.5 million people over the past two months alone. A significant fraction of these deaths results from diseases like emphysema/COPD, lower respiratory infections, and cancers of the lung and airway whose symptoms are clinically indistinguishable from the nonspecific symptoms seen in severe COVID-19 cases. And, perhaps unsurprisingly, the death rate from COVID-19 in China spiked precisely among the same age groups in which these chronic diseases first become common. During the peak of the outbreak in China in January and early February, around 25 patients per day were dying with SARS-CoV-2. Most were older patients in whom the chronic diseases listed above are prevalent. Most deaths occurred in Hubei province, an area in which lung cancer and emphysema/COPD are significantly higher than national averages in China, a country where half of all men smoke. How were doctors supposed to sort out which of those 25 out of 25,000 daily deaths were solely due to coronavirus, and which were more complicated? What we need to know is how many excess deaths this virus causes.
This is where the Diamond Princess data provides important insight. Of the 3,711 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 four.