Archived Facebook Coronavirus Posts: March 2nd

[I'm transitioning a bunch of Facebook posts I made on the Coronavirus to my blog.  This was from March 2nd]

(Warning: musing on fatality statistics and the coronavirus follows. Not necessarily edifying material . . . though I suppose it is true that Catholics are urged to consider the Last Things during Lent.)

I've been reading on how Case Fatality Rates (CFR) are calculated in order to try to figure out whether Covid-19 is actually going to be worse than the seasonal flu or not, and roughly how much if so. The CFR is defined as how many people who are infected will end up dying. For the seasonal flu, it's surprisingly difficult to calculate, but I see numbers *in the US* generally from 0.02% to 0.08%. I've also seen a lot of people throw a "0.1%" number around, but I think that's a *worldwide* estimate. Death rates vary substantially according to quality of medical care in these cases.

The important factor that makes this difficult to judge in the real world is the difference between what you might define a "case" to be. In the early stages of a pandemic, usually the only way to define a "case" is either "someone admitted to a hospital with the right symptoms" or "someone who tested positive for the disease". In *either* case, this will inflate the actual fatality of the disease because it will not count those people who came down with the disease in a milder form and did not seek admission to the hospital, nor ever get tested.

In studies that have calculated the CFR from hospital admitted cases, we have seen estimates ranging from just under 1% up to 5%, but most people think these numbers will get smaller after all infected and not just hospital cases are considered. Can we attempt to do that now?

One approach would be to use the Diamond Princess cruise passengers as guinea pigs. In this case, with a captured audience that was under intense scrutiny, I think we can be confident that we actually know all the people who were infected. Everyone was tested and nobody who got sick in that situation would have shrugged it off as "no big deal". So what do the numbers that we know from the Diamond Princess cases tell us?

As of today, 6 out of the 705 people who have been confirmed to have caught the disease on the ship have died. Assuming all infected people were caught, this makes for a CFR of about .8. *But* we also need to correct for another factor, which is age. Cruise ships have a higher proportion of elderly passengers compared to the general population. My methods here are not very exacting (basically I found a graph of passenger age distribution on cruise ships and a graph of the general age population and squinted at them side-by-side), but I would estimate that cruise ships have 2-3 times the number of the crucial >70 age group compared to the general population. At least 5 out of the 6 people who have died so far from the Diamond Princess were older than 70. So in estimating the CFR from the Diamond Princess, I would divide by 3, and then adjust up a little bit more to account for the fact that there are still some passengers sick who may yet die. This gives me a real-world CFR of about 0.3%.

Two big problems with this estimation strategy: First, this is a small sample size. We really shouldn't put too much weight in what we can learn from a population of under 1000 patients. Second, as mentioned there are still people in this population who have not completed the course of the illness. So it is quite conceivable that another 20 people could still die from this group, which would dramatically change the final CFR.

Supposing, though, that 0.3% is a correct CFR for Covid-19. How many people would we expect to die from this illness if it becomes epidemic in the United States? This is also very difficult to determine, because we know even less about the total number of infected the coronavirus is likely to cause than we do about its fatality rates.

The seasonal flu is estimated to infect between 3% and 11% of the United States population each year. The new coronavirus is liable to infect more people: it has been estimated to have a base infectivity rate well over double that of the seasonal flu, it spreads asymptomatically or close to it, there is no "herd" immunity to the virus in the population, and it is unlikely to be slowed by a change of seasons (this normally happening to viruses *after* they become endemic in a population and not during their first introduction to the scene). So I think a guess of 15% of the population being infected is pretty reasonable.

This works out to a total death rate in the United States of about 150,000 people this year, overwhelmingly from the >70 year old population, but hitting a fair amount of younger people as well.

One important caveat to this number: the difference between a 1% hospitalized CFR and 5% CFR in China is largely between the Hubei province and everywhere else in China. To the extent you trust these numbers, the natural corollary is that as hospital systems get overwhelmed, as happened in Hubei, the fatality of the disease increases as the ability to care for the critical cases goes down. So *if*, in the United States, we start to see hospitals overwhelmed with critical cases, we could see the death rate go up to something closer to the 500-600 thousand mark.