Herd Immunity

Herd Immunity

by Alan E Gorlick

When will Covid be over?  Are we close to herd immunity?  What is herd immunity anyway?

We know that herd immunity has a great resume.  Among its conquests are smallpox, measles (well almost, until the anti-vaxxers), rubella, and giant steps toward getting rid of polio.

We also know that the world’s supply of the novel coronavirus, the culprit behind Covid, wouldn’t fill a coffee mug.  Yet this tiny bug packs quite a wallop.  So getting rid of it is high on our list.  Herd immunity seems to play a role in the fight.

We are told that we can finally exhale when 60% of us are fully vaccinated or Covid survivors.  No wait, that number is 70%.  Did we say 70%; sorry we meant 72.5%.  Why can’t experts agree with each other, or even with themselves from last week?

To appreciate why this is so difficult, we need to understand the process.  A highly contagious virus such as corona needs to jump from one host to another in order to survive.  If there isn’t a receptive host available, the virus that is shed will simply disappear.  This is different from bacteria which can live for eons on their own.

We rmeasure contagiousness with a metric known as R-nought.  (When did we start saying “nought” again?) The notation is R₀. The number represents how many new people, on average, are likely to catch Covid from each sick person.

A fairly communicable virus might have an R₀ of 2.5.  Even if we know that number for sure, and we don't, we realize that each sick person cannot exactly infect 2.5 others.  So, did we mean that every other sick person infects 5 new people, or that 1 person in 10 infects 25 others at a super-spreader event?  The solution to the first might be a temporary lockdown, to the second would be banning large crowds.

The formula for herd immunity is:

percentage = (1 - 1/R₀)

So, if R₀ is 1.5, the percentage for herd immunity would be a quick 33%.  If it’s a more likely 2.5, the percentage jumps to 60%.  If R₀ is really 3.5, then we have to cover 71%.  As experts try to determine R₀, it’s easy to see why herd immunity is a moving target, as well as a goal that can vary from one part of the population to another.

For the calculation to have any meaning, we also have to assume that vaccines work the same for everyone, that inter-personal contact is random, that we get the next round of booster shots before the initial protection fades, and that vaccines reduce transmission while protecting the recipient.

Nevertheless, let's assume we do get to herd immunity.  What then?  While we might bristle at being part of a herd, immunity sure sounds good.  How good is it?  Unfortunately, unvaccinated people could still get Covid.  Sick folks would still shed bits of virus, but those nasty germs will have fewer new homes.  So rather than each sick person infecting more than one, each will infect less than one.  Thus over time the virus will die out.  It will not disappear overnight.

So, if we want to knock out Covid, herd immunity could eventually do the job but it’s messy, time-consuming, and could leave us vulnerable to a spike causing a setback.  Even if we reach that goal, we shouldn't stop there.  Get the vaccine.

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Originally published  in "Well Being" magazine, July/August 2021.  Reprinted with permission.