26 December 2020 Blog Post: What Is Happening in Los Angeles? Looking at Prevalence and Why It Should Scare You

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.

The current prevalence in Los Angeles County stands at 16.6%.  This means that 16 out of every 100 individuals in our County right now, as you read this, has COVID-19.  Has it as in they are actively infected, shedding virus and able to infect others.  Get 10 people together, two people have it.

During the summer “peak” of cases, the prevalence peaked at 2.6%.

The biggest predictor of an individual’s risk of acquiring COVID-19 is the population prevalence.  This current number of 16.6%  should scare you.  The slope of the prevalence curve  which has absolutely skyrocketed since mid-November should scare you.  The fact that there are two approved available vaccines that are not being distributed widely or rapidly should scare you as vaccination is our only way out of this mess.

These data – should scare you.

𝗦𝗶𝗴𝗻 𝗨𝗽 𝗳𝗼𝗿 𝗢𝘂𝗿 𝗡𝗲𝘄𝘀𝗹𝗲𝘁𝘁𝗲𝗿

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