How close will the U.S. be to herd immunity for COVID-19, on March 1 2021? Even with the significant uncertainty involved, it’s worth making a few simple projections of that. If nothing else this will put the current vaccination effort into perspective.
Even under a rosy scenario where vaccine injections double from the current 1M/day, and new infections continue to fall, only about half the population will be immune to COVID-19 as of the start of March 2021. That’s well below the 70% that was thought to be required for herd immunity under the original strains of COVID, let alone the higher percentage that will be required for the more infectious U.K. strain.
Why does March 1, 2021 matter? If the U.S. COVID third wave truly has crested, our next test will come as the more-contagious U.K. variant becomes the dominant U.S. COVID strain. That’s predicted to happen sometime in March 2021. Near as I can tell, that prediction is based solely on the amount of time it took for that strain to become dominant in the London area. So that amounts to a crude guess. But, at present, that’s the only guess we’ve got.
And if that’s correct, then vaccinations should proceed as quickly as possible. But under any plausible scenario, vaccination alone won’t prevent a fourth U.S. wave of COVID-19. We really need to be thinking about what else we’re going to do — such as making N95 masks available to citizens — in addition to vaccinations.
Details follow.
Projecting percent of population immune under two vaccination scenarios
I want to be clear that what you see below are projections, that is, a what-if scenarios. The only guarantee is that this is a reasonably accurate calculation, given the assumptions. These aren’t predictions. They aren’t assertions that the numbers shown here are going to come true.
Both projections below start from today’s figures from CDC. Both assume five total COVID cases for every diagnosed case (Post #940). Both assume that the current rate of decline in daily new cases (roughly 18% per week) continues, leading to an average of just 18 new cases/ 100,000/ day on March 1, 2021.
These projections also assume a split of the vaccine doses between first and second shots, but it hardly matters. In essence, you can have twice as many people be half as well protected (single shots), or have half as many people be twice as well protected (two shots). The choice between the two hardly matters in terms of total population immunity.
Both assume that the overlap between the infected populations and the immunized populations occurs at random. (As noted in earlier posts, the CDC suggests that you get the vaccine even if you have been infected, something I have characterized as inefficient.)
Throughout, detail may not add to totals due to rounding.
Baseline calculation.
As of today, I estimate that about 41% of the U.S. population is now immune to COVID-19. That’s almost entirely due to the estimated 38% of the population that has already been infected.
Scenario 1: 1M vaccine shots per day through 3/1/2021
The first scenario assumes that the Biden administration and state health departments merely keep up the current pace of 1M injections per day. Under that assumption (and the assumptions noted earlier), around 49% of the U.S. population will be immune to COVID-19 on 3/1/2021. If the CDC could (or would) give vaccines only to those who had not already been infected,that would rise to 53% (the “duplicated total” line).
If you look at the difference from the baseline, new infections still account for the majority of newly-immune individuals. Of the total increase of around 9%, 6% arise from new infections (right column, third line). Immunizations add a further 3%, once you account for the overlap between the infected and immunized populations.
Scenario 2: 2M vaccine shots per day through 3/1/2021
The second scenario doubles the rate of vaccination. That is, I think, hugely optimistic, given the timeframe. In that scenario, around 52% of the U.S. population will be immune to COVID-19 on 3/1/2021. If the CDC could (or would) give vaccines only to those who had not already been infected,that would rise to 59% (the “duplicated total” line).
If you look at the difference from the baseline, under that scenario, new infections and vaccinations play equal roles in expanding the COVID-immune population. Of the total increase of around 12%, 6% still arise from new infections (right column, third line). Immunizations add a further 6%, once you account for the overlap between the infected and immunized populations.
A few simple conclusions
If current trends continue, only about half the U.S. will have COVID-19 immunity by the time the U.K. strain is projected to start to be the dominant COVID strain in the U.S.
It’s not clear that this fact, by itself, guarantees a fourth U.S. COVID wave. But I think that would be everybody’s educated guess, unless some new and effective means of blocking the virus come along. Such as issuing N95 masks to citizens. Or going into a stricter form of lockdown.
Why? Based on the reported observed reproduction number (“R-value”) for the new strain, its greater infectiousness would be more than enough to turn the current decline in cases into an increase, absent new action.
Detail of calculation: Best guess, it takes an average of 4.5 days for every new generation of COVID infections. (That’s the average time between when a person gets infected, and the time that person infects someone else). Under that assumption, the current (roughly) 18%/week reduction in new COVID cases corresponds to a current R value (reproduction number) of around 0.88. But the new U.K. strain is reported to increase R by about 0.5. If we layer the U.K. strain on top of the existing declines, that would push the R value above 1.0, and new cases would begin to increase again.
So, vaccinations should proceed with all due haste. But they aren’t likely to prevent a fourth wave. Not if the U.K. variant spreads as expected, and is as infectious as it has been estimated to be. All we can do is expect it, plan for it, and not be surprised when it happens.
Now might be a really good time to think about what you’ll do, if you’re in the half of the population that isn’t immune at that time. Other than moving to North Dakota, which has the best chance of anywhere in the U.S. of having reached herd immunity by then. Maybe right now would be a good time to upgrade to the best mask you can get. And then ponder what else you’d be willing to do if and when the U.K. strain sets off a fourth wave of COVID this spring.
