Herd immunity isn't magic

Individuals who are in whatever degree immune to X are so because their immune systems know how to do antibodies, generally either because the individuals were previously ill with X or because they have been vaccinated against X.

As the number of people in the herd who have such immunity rises the chance of exposure drops for any individual in the herd, including those without immunity.

So the chance of any given individual who is not immune catching X drops

That is herd immunity.

That fact didn't stop somebody on cable news just now saying that as the number of people in the herd who have immunity rises that immunity transfers to others somehow.

The Brit government understands that, but has made a weird choice, all the same.

When I heard about Britain’s ‘herd immunity’ coronavirus plan, I thought it was satire

A large proportion of the population is at lower risk of developing severe disease: roughly speaking anyone up to the age of 40. 

So the reasoning goes that even though in a perfect world we’d not want anyone to take the risk of infection, generating immunity in younger people is a way of protecting the population as a whole.

We talk about vaccines generating herd immunity, so why is this different? 

Because this is not a vaccine. 

This is an actual pandemic that will make a very large number of people sick, and some of them will die. 

Even though the mortality rate is likely quite low, a small fraction of a very large number is still a large number. 

And the mortality rate will climb when the NHS is overwhelmed. 

This would be expected to happen, even if we make the generous assumption that the government were entirely successful in restricting the virus to the low-risk population[.]

. . . .

And of course you can’t restrict it to this age group. 

Think of all the people aged between 20 and 40 who work in healthcare, or old people’s homes. 

You don’t need many introductions into settings like these for what we might coyly call “severe outcomes”. 

In Washington State, nearly all the deaths reported so far have been associated with nursing homes. 

Is everyone in a high-risk group supposed to withdraw themselves from society for six months until they can emerge once the (so far entirely imaginary) second wave has been averted?

About that second wave: let me be clear. 

Second waves are real things, and we have seen them in flu pandemics. 

This is not a flu pandemic. 

Flu rules do not apply. 

There might well be a second wave, I honestly don’t know. 

But vulnerable people should not be exposed to a virus right now in the service of a hypothetical future.

. . . .

The UK should not be trying to create herd immunity, that will take care of itself. Policy should be directed at slowing the outbreak to a (more) manageable rate. 

What this looks like is strong social distancing.