# 3. COVID-19 Prevalence and Incidence

(3/17/20) Based on the analyses described in my previous post (below) and updated to today’s latest World Health Organization (WHO) data, I can make the following estimations for three time points, 3 weeks ago, today and 3 weeks from now:

You’ll need to bear with me as I explain this.

• Recall that I am taking cumulative and weekly rates of death to surmise the prevalence (total cases) and incidence (new cases/week) for 3 weeks ago (2/23/20). That is because I assume that anyone who dies, contracted the illness on average 3 weeks earlier.
• So on 2/23/20, I predict that there were 8,500 people in the US with COVID-19 (assuming a 1% mortality rate). Reports would have said less than 1,000, but plenty are missed for lack of testing.
• I then calculate the prevalence and incidence by fitting a polynomial function (n=2-4 to get a good fit) the growth rate for death (see plots below or take my word for it). The polynomial fits to the curves have uncertainty so I provide low and high estimates. The plots and functional fits (for the high estimate cases) are shown below and complement the plots I emailed on 3/14 (further below).
• So today I predict that there are 48,000 – 130,000 cases of COVID-19 in the US. The documented number is several thousand, but again a gross under-counting, so I totally ignore those pronouncements and why I started my own modeling.
• So the US is at a very low prevalence relative to the population of 350,000,000, however, the growth rate is quite high and accelerating, so we need to exercise social distancing.
• I also predict total deaths (low and high estimates) for the US, WW and other countries for 3 weeks from now based on the polynomial fits.
• China and S. Korea, if the WHO database is to be believed, have done a remarkable job of containing COVID-19.
• Italy is by far the most scariest scenario. The death rate is accelerating, but that reflects an acceleration of the prevalence about 3 weeks ago, so we will not know how effective the aggressive government intervention is until we see a rolling off of the death rate, which as I said is about a 3 week lagging indicator.
• Same goes for the U.S. The death rate is growing at about the same rate as Italy (75% of the total deaths in just the last week). However, that is working from low numbers in the US. High estimates for prevalence of COVID-19 are about 1/3000 for Americans, but for Italians an alarming 1/30. So we are in better shape by a factor of about100x.
• Now there are a lot of assumptions in my model, e.g., I am not accounting for recovery, but if the growth rates are accelerating that will not take things down much.

The reason I did this modeling is because of all the suspect reporting out there about total confirmed cases, which are meaningless because of gross under-counting. Further the growth rates are more due to increased testing than to actual incidence. Further I have seen some doomsday claims of hundreds of thousands of infections in the US based on nothing but conjecture. Finally the “professional” epidemiologists and modelers are getting way to complicated using to many ill-defined variables. In fact the only statistic that matters is death. That’s usually an accurate number.

I will be following the death trends and hope we see a rolling off in the US and elsewhere in the world. If Italy gets a grip on their problem, then we can be assured that the rest of the world will be OK. Hard to say what would be considered sufficient tapering off to relax social restrictions. But if China and S. Korea are any indications it could be as soon as, but not less than, 1 month. Still I’m cautiously optimistic about the prospects for the U.S. and WW.