Natural rates of infection and the optimum level of simplification

How much dumbing down is good for our health?

Keith S. Taber

I just heard the UK Prime Minister introduce a public information message about what would be considered when deciding to ease current measures to tackle the COVID-19 pandemic.

Two particular statements in the clip played gained my attention:

• "All viruses, like normal 'flu, have a rate of infection. Scientists call this R. R is the average number of people one infected person passes the virus onto."
• "In March, at its peak, R was around 3, which seems to be the natural rate for this virus."

Neither of these statements seemed strictly correct to me.

To make things clearer, let's call a spade, a fork

Surely the rate of infection is the number of people who are infected in a unit time period – say per day. R is something else – the reproduction number. Now, those working in the public understanding of science, just as in science education, have to seek an optimum level of simplification when communicating with non-experts. There is no point using complex language that will be unclear to people, and so likely lead to them disengage with the message. So, simplification may indeed be needed. But not such a degree of simplification that what we say no longer adequately represents the ideas we are trying to convey.

But the term 'reproduction number' is not some really obscure and inaccessible jargon – it uses words that most people are quite familiar and comfortable with.It does not seem any more technical and frightening that the term 'rate of infection'. Now I accept that perhaps the compound phrase 'reproduction number' is itself unfamiliar, whereas 'rate of infection' is more commonly heard. BUT rate of infection already has a meaning, a different one – so is it sensible to confuse matters by defining rate of infection with a new meaning inconsistent with the existing common usage?

This seems an odd way to promote public understanding of science to me. This is a bit like deciding that the term 'electrical field' may seem a bit too technical for an audience, so it will be a good idea to instead start calling it 'gravity' from now on, because people are used to that term. Or thinking that 'water of crystallisation' sounds obscure, so deciding to refer to the copper sulphate crystal incorporating 'ice cubes' when talking to non-experts because they know what ice cubes are (i.e., something other than molecules of water of crystallisation!).

So what is natural about rates of viral infection?

I was not sure precisely what a normal 'flu was (in relation to an abnormal 'flu, presumably?), but was more surprised to the reference to a virus having a natural rate of infection – even if this actually meant a natural reproduction number.

Will this not depend on the conditions in which the virus exists?* R will surely be very different in a population that is sparsely spread with small social group sizes than in a population that is largely living in extended family groups in overcrowded slums – so what is the natural environment for that virus?

We have reduced R by social distancing and increased hygiene measures. Are we to assume what is natural is the work and social (and hygiene) habits of the UK population as it was in February 2020, rather than now? If so, were the social conditions in the UK in 1920 or 1820 'unnatural'? So, I think the reference to the rate (actually R) being 3 is not a natural rate, but the R value contingent on the specific conditions of UK social and economic activity at a particular historical moment. To believe that the way WE live NOW (or, actually, two months ago) reflects what is natural seems a very anthropocentric notion of 'natural'.

I guess I am being pedantic (one of the few things I tend to be good at – and we all need to work to our strengths) but it seems to me that if you are going to commission a public health message at a time when the public understanding of science is actually a matter of life and death, then it is worth trying to get the science right.

* This seemed intuitively obvious, but I thought I ought to check. A quick web-search led to lots of different estimates of R (or R₀, that is R whilst a population is all susceptible) presented as if there was a single right value (even if we do not know it precisely) that applied across different contexts globally. Hm. So, I was reassured to come across: "Firstly, R₀ is not an intrinsic variable of the infectious agent, but it is calculated through at least three parameters: the duration of contagiousness; the likelihood of infection per contact between; and the contact rate, along with economical, social and environmental factors, that may vary among studies aimed to estimate the R₀", Viceconte and  Petrosillo, 2020, COVID-19 R₀: Magic number or conundrum?, Infectious Disease Reports, 12(1).