The formula for HIT (herd immunity threshold) is 1-1/R0.
In trying to find the R0 for our state I finally stumbled onto the University’s dashboard for tracking the Virus. I’m impressed. I’m mean I really impressed. University of Tennessee. Go figure. Way to go guys.
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ArcGIS Dashboards
Anyway, the mean value for R0 in Tennessee right now is 1.1 so plugging that number into the formula we get 1-1/1.1 = 0.091 which means to hit the herd immunity threshold in our state we need 9.1% of the population to be immune. Our population is around 6.9 million so that implies we need around 627k people to be immunized to reach herd immunity.
As per a previous post, we know from the CDC that the number of cases reported is much, much lower than the number of cases in actuality. They determined that this was the case all across the country by data collection and antibody surveys in various regions. They settled on an estimate of actual cases being 10 times higher than reported cases but the number varies from place to place. One survey in Missouri indicated the number of actual cases, people with antibodies to the Covid-19 virus, was 24 times the number of reported cases in that region.
If we use the CDC’s estimate of 10 that means we may have 10 times the number of reported cases (71,540), which would put us at 715,400 cases in Tennessee. That means we should be in the range of the herd immunity threshold for our state. It probably doesn’t help that we don’t have a wall or even a fence around our state so we have a lot of people coming and going but still the susceptible native population numbers should be declining.
I think another way, and maybe the CDC is doing this, of projecting the actual number of cases is by using the IFR, infection fatality rate. The case fatality rate (CFR), number of deaths/number of REPORTED cases, doesn’t really tell a whole lot. The IFR is different for different age groups and it can vary a lot by region and a number of other variables, including the quality of post-infection medical care. The latest number I’ve seen is that the IFR nationwide is presently around 0.26% and may decline further. If we divide the current number of fatalities in our state by 0.0026 we can get a rough estimate of what the actual number of infections might be/have been. That would be 796/0.0026 = 306k.
Most immunizations (including vaccines) have a shelf life. Most are from a few years up to several years, I think 18 years in the case of the measles, 4-5 years I think is typical for a given strain of the seasonal flu virus. All my life I’ve been told to get a tetanus booster every 10 years. Anyway, the thing is I don’t know of any situation where the shelf life of an immunization is less than 4 years. jmo.
Admittedly, there is a glitch with this analysis. There are a number of things which can create a variance in the value of R0, population density, social distancing, wearing a face mask, sanitation, etc. These variables convert R0 to Re (R effective) so the HIT calculation above might be grossly understated. The initial widely accepted value for R0 for the Covid-19 virus was 2.5 so if we plug that into the HIT equation we get 1-1/2.5 or 60%. If we need 60% of the state to have immunity then that number would be 4.14m so we’d have a long ways to go. Almost everything I have read of late says the number is far lower than initially thought. The most optimistic numbers say, and again this varies by regions and other non-artificial variables, that HIT can be reached in some areas with less than 20% immunization. My own estimate, which may be grossly in error, for our state is we need to get to 16.67% or 1.15m actual cases to achieve HIT in Tennessee and if I am anywhere in the ball p;ark we should already be well over half-way there. I expect the daily reported numbers to be dropping rapidly by the time we reach August 1st. jmo.
