Update: Herd immunity forecast

We’ll reach the promised land by early June … maybe earlier!

That’s later than Dr. Makary (“We’ll Have Herd Immunity by April”.) … but way sooner than Dr. Fauci, who’s still pitching Christmas (or just before the 2022 elections).

In prior posts, we’ve worked through the logic of our simple forecasting model.

See Herd Immunity: By the Numbers

Our foundational premise: People develop immunity to COVID in 2 main ways: (1) by surviving a COVID infection and developing “natural immunity, or (2) by getting vaccinated.

So, if those 2 groups add up to 200 million (80% of the 225 million adults 18 & over), we’ve reached the promised land: herd immunity.

Statistical note: There is some overlap between the 2 groups, namely people who get vaccinated even though they have survived Covid infections and likely have natural immunity.  Our forecast adjusts for the overlap.

Based on our assumptions (which are delineated below, after the table) and our calculations, we’ll most likely get to herd immunity before summer.

Specifically, in May in our likely scenario (think: 80% likely) … early June in our highly likely scenario (think: near certainty)


Below are our key assumptions & calculations


Key assumptions & calculations:

> Relevant population (line 2): 250 million adults 18 & over … excludes under 18 since they are minimally susceptible to Covid and are not authorized for vaccines

> Herd immunity target (lines 3 & 4): 200 million (80% of the 225 million adults 18 & over); 80% is Fauci’s current proclamation (though he keeps upping it as we get closer to the goal line).

> VAX 1st shots (line 5): As reported by the CDC. Assumes that people only require the 1st shot of Pfizer / Moderna to be substantially immunized (roughly 80% effectiveness); J&J only requires one shot (70% effectiveness).

> Confirmed cases (line 6): As reported by the CDC.

> Detection rate (line 7): IHME estimates the true number of infected individuals in the US is roughly 2 to 4 times higher than the number of reported (aka. “confirmed”) cases.  Said differently, only 25% to 50% of all infections are detected. The vast majority of undetected cases (i.e. untested and unreported) are asymptomatic infections.

> Total Pre-Infected (line 8): The total of already confirmed cases and the estimated number of unreported cases (i.e. those not detected or confirmed by testing).  All of these people are presumed to have developed “natural” immunities.

> Total Immune (line 9): The sum of people vaccinated (with at least 1st shots) and the total number of people previously infected with the virus. This total assumes that no previously infected people have been vaccinated.  This assumption is relaxed in subsequent steps.

> Base # of vaccinations required (line 10): This is the minimum number of 1st shots required to reach herd immunity.  It assumes that no previously infected people have been vaccinated.

> Pre-infected people who get vaccinated (lines 11 & 12): In the likely case, it is assumed that 50% of all previously infected people get vaccinated (even though they have natural immunity from their infections). In the highly likely scenario, it is assumed that all previously infected people get vaccinated.

> Net # of vaccinations required (line 13): The sum of the base # (which assumes that no pre-infected people get vaccinated) and the number of pre-infected people who do get vaccinated.

> Shots per day (line 14): Estimated current running rate of vaccinations adjusted for the increasing inclusion of the J&J vaccines.

> Percentage and number of 1st shots (lines 15 & 16): Assumes that Pfizer & Moderna shots are split evenly between 1st and 2nd shots … and that all J&J are 1st (and only) shots.

> Days until herd immunity (line 17): The gross number of vaccinations required divided by the number of shots per day … adjusted for the mix of 1st shots and 2nd shot mix.


Stay tuned for periodic updates…


DISCLAIMER: I’m not a medical professional or scientist — just a curious, self-interested guy.  So, don’t take anything that I say or write as medical advice. Get that from your doctor!

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