Wednesday, March 18, 2020

COVID-19: Some US estimates -- the knife's edge

My lower bound on current number of cases in the US (March 18): tens of thousands. Positive test results are just the tip of the iceberg.

The unconstrained doubling time is roughly 3 days. Some recent estimates of this timescale by country (note these are trailing estimates): Japan is ~10 days, S. Korea ~16, PRC ~36 days currently. Most European countries are at 3-4 days, although these estimates are unreliable as the increase is mainly due to wider testing and does not capture growth rate very well.

In Michigan we've closed K-12 schools and universities, as well as bars, restaurants, gyms, etc. I would guess that the doubling time has been increased to ~7 or 10 days by these measures.

Medical system overload will happen for sure at ~1 million total cases in the US, based on 5% of cases requiring intensive care, and total ICU capacity (~50k) in the US. Local regions may go into crisis much earlier, in particular dense urban regions that are more cosmopolitan than the US as a whole. The threshold is roughly 0.3 percent of population infected -- only a few per thousand!

Overload may happen much earlier -- hospital systems run at over 90% occupancy. So most spaces are already filled by sick people. The numbers above might be optimistic by an order of magnitude -- perhaps 100k total cases in US, or 0.03 percent of local population infected will lead to at least some health system crises.

Another factor contributing to early overload is exhaustion of protective equipment for medical personnel (already starting to happen in some places like WA) and eventual sickness of those personnel.

Best case: Widespread US crisis after another (3-5) doublings:  2^3 to 2^5 more cases. Assuming the whole country adopts isolation measures as in Michigan: roughly 30 days until widespread crises. I suspect we will see localized crises much sooner.

How might things turn out better?

Warm weather plus isolation measures push doubling time to 14 or even 20 days. Even in the best case this won't apply throughout the whole country, so still expect problems in some regions.

Earlier posts: COVID-19 Notes , COVID-19 Update: Developing Countries and Flattening the US Curve.

The figures below are from this NYTimes article describing a Harvard analysis similar to the one above. Note in the first figure the solid yellow line is crossed in all scenarios very early on -- i.e., +1 or 2 months. Overloaded ICUs are likely in the near future.

No comments:

Blog Archive