Wednesday, April 22, 2020

Why low estimates matter

This is Joseph

People wonder why I am so concerned about the preprints that are suggesting much higher levels of infection (and thus lower fatality rates) than expected. Consider this example:

The low estimate of 500 was used to justify a policy response to the covid-19 epidemic. The current number as of April 20th is 45,013 reported US deaths.

But we need to be careful about predictions that can change. This study suggests a lower infection fatality rate then previous studies show:
We can use our prevalence estimates to approximate the infection fatality rate from COVID-19 in Santa Clara County. As of April 10, 2020, 50 people have died of COVID-19 in the County, with an average increase of 6% daily in the number of deaths. If our estimates of 48,000-81,000 infections represent the cumulative total on April 1, and we project deaths to April 22 (a 3 week lag from time of infection to death), we estimate about 100 deaths in the county. A hundred deaths out of 48,000-81,000 infections corresponds to an infection fatality rate of 0.12-0.2%. If antibodies take longer than 3 days to appear, if the average duration from case identification to death is less than 3 weeks, or if the epidemic wave has peaked and growth in deaths is less than 6% daily, then the infection fatality rate would be lower. These straightforward estimations of infection fatality rate fail to account for age structure and changing treatment approaches to COVID-19. Nevertheless, our prevalence estimates can be used to update existing fatality rates given the large upwards revision of under-ascertainment.
 Now, we all do "back of the envelope" calculations. This is me doing some. But we need to be careful. In a press release, the senior author pointed out that this calculation made the IFR of covid-19 about that of the flu. Like these are actual headlines. But this would be a massive issue, if true, as the decisions we are making depend on a higher IFR. But there are concerns with this research and there is a second study that appears to be only via press release.

Now, let me be clear, I would be deliriously happy if these studies were correct. I would feel much better about loosening the lock-down and "taking it on the chin", as Boris Johnson said. These super low rates of fatality would shift the conversation about the economy, as well as suggesting people will go back to movie theaters because we'll quickly all be immune.

But these numbers don't seem compatible with New York, Italy, Spain, or the careful studies in Iceland. Corrections to the Reason story note the NY problem. This doesn't mean we don't have a lot of asymptomatic infections, we do.

But it is important that we not base policy on numbers that can be rapidly revised and quite different when they are eventually put into the record. The reason I started with the article above is that it is easy to dismiss the epidemic. I would like there not to be one too! But if you are going to argue, either way, in an official capacity then there should be some serious accountability if the estimates are way off, in a way that a statistician helping might easily fix.

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