Post #1035: Herd immunity, part 1: Vaccines don’t matter, much, yet

Posted on February 27, 2021

It’s about time to revisit herd immunity.  So far, all I’ve been able to say, from the data, is that we’re not there yet.  (Duh.)

I’m going to start off this next set of posts by explaining why the current vaccinations don’t matter (much, yet) in terms of getting individual states or the U.S. as a whole to the herd immunity level.

Why vaccinations (mostly) don’t (yet) matter for herd immunity.

First, I’m not a nut.  I’m signed up to get the COVID-19 vaccine.  I think everyone should get it.  I think vaccines rate right up there with flush toilets and running water in terms of great leaps forward in public health.

Instead, this is just basic science.  And it’s an aspect of herd immunity that seems to have been overlooked by almost all the popular press.  It’s not rocket science, it’s just formalizing my post on why you should have invited Grandma to your Super Bowl party (Post #999).

If the goal is herd immunity, we’re vaccinating the wrong people first.  Mostly.

Herd immunity works because immune individuals stop chains of infection.  They are “duds” from the standpoint of spreading the disease.  And so, vaccination only has an impact on herd immunity if you vaccinate people who would otherwise have gotten infected.  If you vaccinate a lot of people who weren’t likely to get infected anyway, it has minimal effect on stopping the further transmission of disease.

The upshot is that if you want to use vaccines to reduce the spread of disease, you focus the vaccinations on the populations who are actively spreading the disease.  (I said it wasn’t rocket science).  In that case, right now, in the U.S., that would be young adults.

Instead, U.S. policy is (largely) focused on vaccinating individuals at high risk of poor outcome — the elderly, and those with comorbidities that (appear to) put them at risk for severe COVID-19.  As well as some classes of workers who are are are thought to be either at high risk of infection, or critical to maintaining economic activity.

To be clear, that was a choice, with forethought. So that’s not a mistake.  But it will definitely have the effect of reducing the vaccine’s effectiveness in slowing the rate of new infections.

In theory, I ought to try to dig up some national data.  But Virginia is a middle-of-the-road state, and Virginia provides the two key tables straight off their COVID-19 dashboards.

Here’s the profile of who’s been vaccinated so far ,by age.  Then a profile of who’s getting infected right now, by age.  (And don’t sweat the cause-and-effect there — the infections by age chart has looks like this for months now.)

So, sure, Virginia has vaccinated a lot of people.  But they haven’t vaccinated a lot of the people who are most active in spreading the disease right now:  Young adults.

Almost half of the 80+ population has been vaccinated.  Which is great for the 80+ population.  But, by and large, that’s not the population that’s out and about, spreading the disease.  Instead, for the critical 20-29 age group, so far, about 7% have been vaccinated.

There’s a second factor, that you can’t see, but that probably plays an even greater role in reducing the effectiveness of vaccines at slowing the spread of COVID-19:  Self-selection.  And by that I mean that the people lining up to get vaccinated early are probably people who are both worried about COVID-19 and cautious about exposing themselves to infection.

In other words, the people pressing to get vaccinated first are probably the people who were less likely to spread disease in the first place.  They are probably mostly from the stay-at-home, employ-proper-COVID-hygiene group.  Or, to turn that around, it’s a fair bet that the anti-vaxxers overlap pretty strongly with the anti-maskers and others who fail to adopt simple measures to prevent spread of disease.

You see this self-selection effect repeatedly in the health services research literature, whenever you look at preventive health care behavior.  You will find that (e.g.) women who get mammograms at the recommended interval are much less likely to be hospitalized for flu.  It’s not that cancer screening repels virus, it’s that people who take good care of themselves do that along many dimensions, up to and including getting the flu shot each year.

In the example above, it’s not cause-and-effect, it’s that compliance with cancer  screening recommendations is a marker for behavior in general.

And so, I would bet that those first in line for COVID-19 vaccine are, by and large, those who are already behaving cautiously.  Which is, once again, exactly the wrong population to target from the standpoint of stopping disease transmissions.  In the same way that, say, distributing condoms to the celibate would have little impact on sexually transmitted diseases.

A final factor that needs to be kept in mind is the long lag between vaccination and full immunity.  It takes about two weeks to get something like 50% effectiveness, and it takes six weeks to achieve the full 90%-ish effectiveness of either of the currently available vaccines.

In fact, one of the ways that you know there’s probably a big self-selection effect is that when somebody gets a COVID-19 infection after being vaccinated, it makes the news.  If you were vaccinating those who were actively spreading disease, that shouldn’t be a rare event.  But in fact, it’s so rare that it makes the news.

(OK, how non-rare should it be?  If the U.S. is vaccinating at the rate of 1.5M shots/day, and infecting at the rate of 20/100K/day, and it takes (say) a week to get any protection at all from the vaccine, if we vaccinated at random, then we ought to be seeing a steady stream of (7 x 1500000 x 20 / 100000 = ) ~2000 people per day who manage to get infected with COVID during the first week following their vaccination.  It should not be even remotely newsworthy.  The fact that an event of that type makes the news tells you that these post-vaccination infections are occurring far less frequently, i.e., these aren’t people who would have been spreading disease absent the vaccine.)


The upshot is that, as of today, the U.S. has vaccinated (partly or fully) just over 14% of the population.  But, for the reasons outlined above, we should expect that to have a far-less-than-proportionate impact on transmission of COVID-19.  How much less, I don’t think anyone can tell you.

All you can say for sure is that this is one of the reasons we’re not seeing any clear evidence of herd immunity yet.  The policy decision was to vaccinate those at high risk of severe disease, and those needed to keep the health care system and the economy working.  And then, to vaccinate individuals in the order that they volunteered for it.

Those are all reasonable decisions.  But as a by-product of that, vaccine has not been concentrated on the populations most responsible for spreading disease.  And so the impact of vaccination on the continued spread of COVID-19 will be substantially less than the simple 14% figure would indicate.