13 December 2022 Blog Post: Observed over Expected

One commonly used statistical tool called the chi-squared test is used to formally evaluate the difference between numbers observed versus numbers expected. Don’t worry, there are no statistics in this Blog post but the concept of Observed over Expected is one that is helpful in framing this season’s risk of COVID-19.

As I prepared this month’s Newsletter, I collated the total number of COVID cases seen by month in our practice (Observed) along with the number of repeat infections (individuals experiencing their second infection). It led to the following graphic which we published in the Newsletter.

It led me to ask the following – what sort of numbers might we expect in the practice if our experience was a reflection of COVID-19 rates in Los Angeles County as a whole?  Performing these calculations gives the following graph:

If we had used incidence rates from the County to estimate an expected COVID-19 case load for our practice we would have massively underestimated our observed numbers (a disaster if we used these to modify staffing levels). The lowest magnitude of difference was in January (8.9 fold underestimate) and the highest was November (239.1 fold underestimate). In November, for instance, County estimates predicted that we would see 0.11 cases when, in fact, we had 27 new COVID cases.

There are a number of potential explanations for these discrepancies, the first being ascertainment. In all likelihood, our patients as a function of having a regular primary care provider and access to testing will more often come to our attention and be diagnosed with COVID-19 than might be expected. In some instances, subclinical or those with even mild symptoms will present for care which otherwise might be missed – particularly since 10.2% of LA County residents lack health insurance entirely.

Is it possible that the patients in our practice are more likely to contract COVID-19 or are simply more susceptible? We do have a population that is somewhat older than the LA County average – about 30% of the practice is over the age of 65 as compared to 15% for the County as a whole. It could be that our patients travel more, work in larger congregate settings or have other characteristics that lead to a higher risk of exposure.

Interestingly, the lowest observed to expected ratio was in January when rapid antigen tests were scarce and individuals were relying on PCR testing to a greater degree. Results from rapid antigen testing stopped being reportable to the County in January of this year as well.

So what does this mean from a practical standpoint? We know that the County numbers are an underestimate of the ‘true’ population incidence. Further, using the County numbers in my own practice is a massive underestimate of what I am actually seeing in the clinic.  Nevertheless, a year over year plot of cases (graph below) shows that 2022 (yellow line) is very much on the same trajectory that we saw in 2020 (blue) and 2021 (red). 

SARS-CoV2 continues to display a clear seasonal pattern – one that becomes obvious even in a context of clear under-reporting of cases. I think it is reasonable to assume that these cases will continue to grow, perhaps not to the magnitude of the Omicron surge we experienced in January 2022, but one that will occupy a significant amount of time and effort in the clinic.

𝗦𝗶𝗴𝗻 𝗨𝗽 𝗳𝗼𝗿 𝗢𝘂𝗿 𝗡𝗲𝘄𝘀𝗹𝗲𝘁𝘁𝗲𝗿

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