The Homa Files

The math says that it’s a stretch, but a real possibility.
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In a WSJ op-ed, Hopkins doc Marty Makary boldly asserted the possibility that “We’ll Have Herd Immunity by April”.

Immediately, Dr. Fauci — our nation’s chief political-scientist — hit the talk shows to hose cold water: “Maybe by Christmas, or mid-2022”.

Note: Mid-2022 is right before the mid-term elections. Hmm.

To calibrate Makary’s logic, I went back to re-read the article and run the numbers…

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.

And, Makary concludes that we’re already approaching herd immunity.

How can that be?

Let’s work the numbers, starting with the herd immunity threshold: How many people have to be immune to achieve herd immunity?

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The threshold target

Dr. Fauci admitted to the NY Times  that — in his role as the nation’s chief political-scientist and arbiter of scientific truth — he has moved the goal posts re: herd immunity – not because of scientific data but because of tangentially-related polling data:

When polls said only about half of all Americans would take a vaccine, I was saying herd immunity would take 70 to 75 percent.

Then, when newer surveys said 60 percent or more would take the vaccine, I thought, ‘I can nudge this up a bit,’ so I went to 80, 85 per cent.

Gotta love following “the science” and “the data”, right?

That notwithstanding, let’s work with the 80% number.

Question: 80% of what? The total population (335 million) or the adult population 18 and older (250 million)?

Children under 18 are 25% of the U.S. population.

“The science” seems agreed that children are minimally susceptible to getting infected and, if infected, are minimally prone to transmit the virus.

And, none of the current vaccines are approved for use on children under 18.

We’ll assume that the 80% applies to the 250 million adults 18 and older.

That sets our herd immunity target at 200 million adults 18 and older (80% x 250 million).

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The vaccinated

Already, about 45 million people have received first doses of vaccine.

There has been a stream of recent evidence that the 1st dose effectiveness of the vaccines is, by itself, very high … over 80%.

Note: Evidence is scant regarding the durability of the induced immunity — i.e. whether it is of short or long duration.

So, we’ll count all 45 million 1st-dosers as immunized.

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Confirmed COVID survivors

Also, we know that there have been almost 30 million confirmed COVID cases.

COVID survivors are generally presumed to be immune.

Makary points out that that reinfections have occurred in less than 1% of people — and when they do occur, the cases are mild.

Again, the durability of the immunity is a big unknown unknown.

So, we’ll count the 30 million confirmed COVID cases as being “naturally” immune.

To be precise, I should subtract the 500,000 confirmed cases that didn’t survive. I didn’t, simply to keep working in round numbers.

That gets us up to 75 million people already having immunities … leaving 125 million still to be vaccinated to reach herd immunity.

At the current vaccination rate (1.5 million each day) it will take about 85 days to reach herd immunity.

Note: Key assumptions are that COVID survivors defer their vaccinations and that all vaccine doses are for 1st shots.

That pushes us to late may or early June.

Still very good … not April, but well before back-to-school in the Fall..

And, there’s a big swing factor that can make the situation even rosier…

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Unconfirmed COVID cases

There’s a very large group of people who are probably immune but oft-ignored.

These are the “hidden” COVID infectees who don’t exhibit symptoms (i.e. “asymptomatic”) and haven’t been tested.

These “unconfirmed COVID cases” are immune, too … right?

How many of them are there?

Makary says that testing has been capturing only from 10% to 25% of infections.

The Institute for Health Metrics and Evaluation (IHME) estimates that the “detection rate” is higher — in the range of 25% to 50%. Source

We’ll assume 25% (the top of Makary’s range and bottom of IHME’s) … which translates to about 3 “unconfirmed” (i.e. asymptomatic) cases for every confirmed case.

Using that factor, there are about 90 million unconfirmed COVID survivors who are also immune. (30 million confirmed cases x 3)

That pushes the number of people with immunities to 165 million.

45 million vaccinated + 30 million confirmed cases + 90 million unconfirmed cases equals 165 million

So, we’re still 35 million short of Fauci’s 80%herd immunity target.

No problem.

At the current vaccination rate (1.5 million per day) it will take about 30 days to reach the herd immunity threshold if all of the shots are 1st shots … and 60 days if 1st & 2nd shots are split 50/50.

That puts us in late April or early May

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Bottom line

Herd immunity doesn’t require that 80% of the (adult) population be vaccinated. Rather, it requires that the sum of vaccinated people and COVID survivors must hit 80%.

Even if we only count the confirmed COVID survivors (30 million) and already vaccinated people (45 million), it will only take about 85 days to reach herd immunity (200 million) at the current vaccination rate (1.5 million each day)

But, that assumes that COVID survivors don’t get vaccinated and that all vaccinations go to 1st-dosers.

There are issues with both of those groups — identification of unconfirmed cases requires testing before vaccinations … and deferring 2nd doses entails risk.

For more detail see VAX: Breaking thru the vaccination bottlenecks…

All things considered, I’m getting more confident that herd immunity is possible … sooner rather than later.

This entry was posted on February 23, 2021 at 10:00 am and is filed under Coronavirus - Covid 19. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.