James Lyons-Weiler, PhD – 3/18/2020
WE KNOW that at any given time during the growth phase of COVID-19 pandemic the number of cases reported represent perhaps only as many as 20% of cases present. We also know that the number of cases we see today are the product of exposure from infected individuals 4-5 days prior.
Therefore, if we use the ratio of the number of cases on a given day, Nx, to the number of cases five days prior (N_(x-5)), we can track the effects of interactions.
Notably, this rough estimator will only be relevant during the growth phase; during retraction, the Effective R0 will become <1 but that could be misleading as it would imply that infected people are preventing infection.
That neat trick is possible via plasma convalescent therapy, but that’s a different but equally important point.
For the outbreak in in the US, the increase since Day 1 looks like this
Now here is the rough Effective R0 estimator for the US over the same time period.
The very simple estimator of the Effective R0 shows that the rate of increase has fluctuated greatly. During times when no testing was being conducted, no new cases were being reported, and thus R0 might also be overestimated later on. Still, it seems useful as an approach to evaluate ongoing interventions such as social distancing and therapeutics.
How to Interpret Effective R0
Effective R0 can be used to get an idea – a very rough idea – on the dynamics of the rate of increase during the growth phase of an epidemic. The goal is to push Effective R0 below 1, and keep it there.
How NOT to Interpret Effective R0
Effective R0 is not R0. It is a product of the outcome of infection, testing, reporting and interventions and other factors.
Don’t Be Complacent. When the R0 start to decline, it is time to continue effective interventions and to add new ones that will push it down even further. Like pulling someone out of the water, you’re not in the clear until they are on dry land. Dry land is Effective R0 = 0/0 — no new cases for a sustained period of time.
Now, let’s push for therapeutics – plasma convalescent therapy and antivirals – because it does not appear that we will defeat SARS-CoV-2 worldwide by Social Distancing alone.