Herd Immunity: By the Numbers

Key: It’s not just people who get vaccinated who are in the immunized part of the “herd”

Earlier this week, we dissected the WSJ op-ed by Hopkins Dr. Marty Makary who boldly claimed  that “We’ll Have Herd Immunity by April”.

Our basic conclusion: The claim isn’t as far-fetched as it sounds. It’s unlikely, but mathematically possible.

The essence of Makary’s logic is that people develop immunity to COVID in 2 main ways: (1) by surviving a COVID infection 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.

The catch: Achieving herd immunity in just a couple of months requires two very bold policy shifts and supportive actions.


COVID Survivors

First, all people who have survived a COVID infection (and developed the associated antibodies) must defer vaccinations.

That’s not as reckless as it sounds since, by definition, those people have already built up some level of immunity.

See Should people who have had COVID get vaccinated?

Of course, execution is a challenge since the majority of COVID survivors — 75% by conservative estimates — were asymptomatic and weren’t tested.

So, how do they know that they were infected, developed the antibodies and can safely defer their vaccinations?

Answer: They would have to take an antibody test before getting vaccinated

COVID-19 IgG antibody testing, also known as serology testing, checks for a type of antibody called immunoglobulin G (IgG).

If a person has been exposed to the virus that causes COVID-19, their body typically produces IgG antibodies as part of the immune response to the virus

If the test is positive for antibodies, the person has some immunity to COVID and should be ok deferring their vaccination for awhile.

Doing so stretches the scarce doses of vaccine to those needing to build some immunity.

But, it’s easier said than done.


Deferred 2nd dosing

Second, a quick build to herd immunity requires that — rather than taking their 2nd dose of vaccine at the currently prescribed 3 or 4 week interval — people getting a 1st dose should defer their 2nd shots until, say, 6 to 12 weeks.

The logic: Evidence is growing that 1st doses provide very high levels of protection … and, some countries are concluding that 2nd doses can be pushed later without jeopardizing the longer-run efficacy of the vaccines.

CDC updates guidance re: 2nd shots:
Data says it’s OK to wait 6 weeks

If 1st-dosers defer their 2nd shot then vaccine is freed up for others to get their relatively high potency 1st shots.

Of course, requires that first-dosers take on some COVID protection risk.

Disclaimer: I’m scheduled for my 2nd shot in a couple of weeks.  I plan to take it … and will squeal like a pig if there’s a policy shift that deprives me of the shot!


Best Case

OK, let’s put the pieces together…

Based on the current levels of confirmed cases and vaccinations, if both of the above policy changes are fully implemented, we could get to herd immunity by mid-April.

Given that we currently have 2 approved vaccines, that’s the proverbial best case.

Here’s the math…


Key assumptions:

  • Target: 80% of adults 18+
  • 25% Covid detection rate;
    75% unconfirmed by testing
  • No Covid survivors vaccinated
  • 1.5 MM shots per day
  • 2nd shots deferred;
    All shots are 1st shots


Worst Case

What if all asymptomatic (and untested) COVID infectees get in line to be vaccinated … and that all 2nd shots are given as prescribed?

That would push the herd immunity time frame out to mid-August.

Statistical note: The additional 133 days (from the best case to the worst case) is roughly evenly caused by each of the two policies.

Key assumptions:

  • Target: 80% of adults 18+
  • 25% Covid detection rate;
    75% unconfirmed by testing
  • 75% of Covid survivors vaccinated
    (all unconfirmed cases)
  • 1.5 MM shots per day
  • 1st & 2nd shots split 50/50


What if J&J is Approved?

J&J has said it would deliver about 20 million doses for U.S. use by the end of March, which may be enough to boost the countrywide capacity for completed vaccinations by about 20%. Source

If true, holding all other factors constant, herd immunity is reached early April in our best case … and end of July in our worst case.


Key assumption change: Shots per day increased by 20% to 1.75 million


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!

One Response to “Herd Immunity: By the Numbers”

  1. Update: Herd immunity forecast | The Homa Files Says:

    […] See Herd Immunity: By the Numbers […]

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