Prevalence is one of the earliest concepts covered in an introductory Epidemiology course is that of prevalence. Defined, prevalence is “the proportion of persons in a population who have a particular disease or attribute at a specified point in time or over a specified period of time.”

Determining the true population frequency of COVID-19 would be a massive, time consuming and expensive undertaking.  It would require public health officials to conduct periodic large-scale surveillance testing via random sampling.  Fortunately, Drs. Chiu and Ndeffo-Mbah from Texas A&M University found that prevalence rate for COVID-19 can be predicted, with a 7-day lag, by the geometric mean of reported case and test positivity rates averaged over the previous 14 days.

So how does this apply to Los Angeles County?  Well I did the analysis – and it’s scary.

RIght now, when you read press reports or watch television or even read this blog, you are inundated with the massive case rate.  But as these numbers rise higher and higher, the practical impact is lost.  Or, as Dr. Naomi Wolf commented on my Twitter – “un-annoted ‘curve’ w/steep upper right is often being used to wave at people’s faces to scare them without data.”  Prevalence is a much more relevant number, and one that illustrates the magnitude of our public health crisis here in Los Angeles.

My intention with Figure 1 below is, in fact, to scare you.  But with data.