What is herd immunity?

We’ve all heard a lot recently about herd immunity (pun intended), but it’s a term which seems to cause a lot of confusion. First off, let me tell you what it’s not. It’s not about a population becoming exposed to the Covid-19 virus and those with some resistance or those being “fitter” surviving while others “less fit” die. This strategy would no doubt leave the survivors with some immunity to Covid-19, but that isn’t what’s meant by herd immunity. I’d also add at this point, although it’s very likely those who have had Covid-19 and survived will gain immunity to the disease, at this early stage we don’t know the extent, how long the immunity will last, or whether it’s possible to suffer the effects of the virus a second time.

To use an analogy, imagine a tinder dry forest and a small fire which starts somewhere near the centre. The fire rapidly ignites one tree and because it’s so close to its neighbour, then that tree also catches fire. The fire spreads from one tree to another until the whole forest is ablaze. Now imagine a forest where some trees are resistant to catching fire. Perhaps miraculous rain clouds soaked some trees and not others – use your own imagination, this is only an analogy in way of explanation. The fire spreads from tree to tree but more slowly, because it can’t get round the soaked trees. If there are a very small number of wet trees, the fire will still rage but it’ll spread at a slower rate. As the number of wet trees increases, there comes a point where the fire burns a few trees but can’t spread any further and burns out. The wet trees protect the dry trees from spread and that’s herd immunity.

Although not exclusively so, the term herd immunity applies to vaccinated populations, where the “wet trees” are those with vaccine immunity, and the dry trees are those unvaccinated. Stopping the spread of a disease does not require 100% of the population to be immune but enough so the metaphorical fire can’t burn round them. Take smallpox as an example. The aim was to vaccinate 80% of the population in infected areas to achieve a herd effect. We irradiated the disease in 1977, although the vaccine uptake was in fact higher than the target 80%.

Epidemiologists talk about the R0 (R-nought), or reproduction ratio, which is the number of people likely to be infected by a single virus in a population with no prior immunity. The R0 for measles, which is highly infectious, is somewhere between 12 to 18 and seasonal flu is around 2. The R0 for Covid-19 is around 1.5 to 3.5.

But here’s the rub. For herd immunity to kick in for Covid-19 you need somewhere around 60% of the population to have immunity. Since we have no vaccine that means 60% of the population need to be infected and recover to gain immunity, which is 36 million people which will lead to tens, if not hundreds of thousands of deaths.

I have no answers I’m afraid because I’m well out my depth with exit strategies. I’m just trying to clarify what’s meant by herd immunity to clear some confusion. In the meantime I think we should trust in the world experts who understand this much better than I do, rather than those claiming to understand herd immunity as a way of getting the economy back on track.

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