The Rules of Contagion: Why Things Spread - and Why They Stop

by Adam Kucharski

According to the SIR model, outbreaks need three things to take off: a sufficiently infectious pathogen, plenty of interactions between different people, and enough of the population who are susceptible. Near the critical herd immunity threshold, a small change in one of these factors can be the difference between a handful of cases and a major epidemic.

Dietz outlined a quantity that would become known as the ‘reproduction number’, or R for short. R represented the number of new infections we’d expect a typical infectious person to generate on average.

If R is 10 in a fully susceptible population, we’d need to vaccinate at least 9 in every 10 people. If R is 20, as it can be for measles, we need to vaccinate 19 out of every 20, or over 95 per cent of the population, to stop outbreaks. This percentage is commonly known as the ‘herd immunity threshold’.

R = Duration × Opportunities × Transmission probability × Susceptibility

Viewing at-risk people as special or different can encourage a ‘them and us’ attitude, leading to segregation and stigma.

As physicist Max Planck supposedly once said, ‘science advances one funeral at a time.’

In the US, the virus spread like a ripple, gradually travelling from the southeast outwards. It took about three months to move 2,000 kilometres across the eastern US, which works out at a speed of just under 1 km/h. On average, you could have outwalked it.

‘The death-rate is a fact; anything beyond this is an inference,’ as Farr put it.

‘You ask me why I do not write something,’ she replied. ‘I think one’s feelings waste themselves in words; they ought all to be distilled into actions and into actions which bring results.’

According to carl bell, a public health specialist at the University of Chicago, three things are required to stop an epidemic: an evidence base, a method for implementation, and political will.

In 1998, the UK switched from selling paracetamol in bottles to blister packs containing up to thirty-two tablets. The extra effort involved with blister packs seemed to deter people; in the decade after the packs were introduced, there was about a 40 per cent reduction in deaths from paracetamol overdoses.

It might be tempting to simply blame patients for taking too much medication, or doctors for overprescribing. But we must also consider the pharmaceutical companies who market strong opioids directly to doctors. And insurance companies, who are often more likely to fund painkillers than alternatives like physiotherapy.

Widespread refusal of vaccination therefore tends to be a luxury, a side effect of living in places that – thanks to vaccination – have seen little of such infections in recent decades.[18]