It’s getting wild out there! Emboldened by Omicron case rates that are imploding (which was completely expected given its rapidity of rise), folks previously quiet about vaccination, mask and mitigation effort mandates are now succeeding in getting these rolled back. While not entirely inappropriate at the population level, it leaves each of us as individuals a bit perplexed. Are these the right decisions? And, more importantly, are these the right decisions for me?
My daughter’s school district announced the end of outdoor mask mandates at the 11th hour (not joking, their email went out at 8PM the night before the change) which gave us no time to discuss with her the change, why it was being done and why it was safe. This after 2 years of telling kids that they themselves were at risk of acquiring the disease and themselves could pass it on to the most vulnerable. They’ve been understandably scared and pulling back on mandates needs careful discussion – even when they are appropriate.
So rather than arguing in circles, I find it most helpful to return to first principles of Epidemiology – which are the concepts of Incidence and Prevalence (which I covered in our February newsletter).
Defined, incidence and prevalence are:
- Incidence (Rate): The number of new cases of disease during a specified time interval per average population during the time interval.
- Prevalence (Rate): The number of current cases (new and pre-existing) over a specified period of time.
Figure 1 below shows the smoothed incidence rate of SARS-CoV-2 infection (presented weekly) in Los Angeles County since March 1st, 2020. While we are well off our peak of 453.19 new daily cases per 100,000 we still have a large number of cases at 34.9. This value above 25 places the County in the “tipping point” of risk levels as outlined by key metrics for COVID suppression set forth by the Brown School of Public Health and Harvard’s Safra Center for Ethics (link: https://globalepidemics.org/key-metrics-for-covid-suppression/). A couple of caveats exist – first, these guidelines were created in 2020 when the virus was novel and spreading in a 100% susceptible population. Secondly, the mortality risks for COVID-19 have changed dramatically since these guidelines were outlined in 2020 given vaccination, boosters and early treatment. Nevertheless, contextualizing the incidence rate in these terms advises caution.
A raw daily rate shows a similar pattern (with less lag as a full week of data is not required). Our current daily rate in Los Angeles County is lower than the smoothed weekly rate at 16.2 new daily cases per 100,000 further illustrating that we are in a downtrend.
Figure 3 below shows the prevalence (presented weekly) of SARS-CoV-2 infection (presented weekly) in Los Angeles County since March 1st, 2020. The estimated prevalence of active infection for the week ending 2/15/2022 is 1.6 per 100 individuals (1.6%). So a gathering of 60 plus individuals representing the County population would, statistically, have at least one positive case. It is important to note that prevalence rate is calculated as a function of the 14 day incidence rate and the 14 day positivity rate. Because it uses the reported positivity rate it led to some inappropriately high estimates during the Omicron surge when many people were testing at home, and not reporting results. For instance, the week ending 1/11/2022 the prevalence estimate was 72.2% which, clearly, is not realistic.
This prevalence calculation is, by definition, a lagging marker. But for those individuals who wish to make conservative activity judgments can be helpful, as the real-time prevalence during a downtrend (as we are in now) will be lower. On the other hand, during an upwards surge, the incidence rate will be a better metric by which to make decisions.
Which leads us to the fundamental question of – what do I do now? Now that there is significant public pressure to remove safeguards such as universal indoor masking, how do I make the best decision for myself in terms of activities and participation?
I find it most helpful to contextualize the incidence rates and prevalence rate. Currently:
Raw Daily Incidence Rate: 16.2 new daily cases per 100,000 population
Smoothed Incidence Rate (by week): 34.9 new daily cases per 100,000
Prevalence Rate: 1.6 active cases per 100 population (1.6%).
Now with this you can start to make some informed decisions (far better than throwing your masks on the doorstep of the LA County Health Department and claiming ‘masks do nothing’). The incidence rates show that Omicron is indeed in a hasty retreat (weekly smoothed rate is more than double the raw and more real time daily incidence rate). And, two weeks ago, the prevalence rate was 1.6% – and is undoubtedly lower currently.
To each their own at this point, but suffice it to say that these numbers and trends make me feel very optimistic about the Spring and a return to normalcy. Personally, now at the two year mark of managing COVID-19 in my practice, I’m not prepared to acquire the infection in what may very well be the 9th inning of the pandemic. So if you see me with my N95 in Pavilions, I’m not virtue signaling, I’m just making the right decision for me…
And waiting for that prevalence rate to drop even further. Maybe at 1 in 50,000 I’d even think about a Dodgers game (Collective Bargaining Agreement aside, that’s a whole separate Blog post).