A Simple Guide to R

Much is said about the R0 or Rz number when it comes to covid-19, but what is R and what is it measuring?

R depends on four factors

1. the duration of time a person is infectious.

2. the average number of opportunities they have to spread the infection each day they’re infectious.

3. the probability an opportunity results in transmission

4. and the average susceptibility of the population

R= Duration x Opportunities x Transmission probability x Susceptibility. (DOTS)

R simply asks: how many people would we expect a case to pass the infection on to?

Apart from measuring transmission rates  from a single infectious person, R can give us clues about how quickly the epidemic will grow.  If R is 2, an initial infected person will generate two cases on average. These two new cases will on average generate two more each, and so on. Carrying on doubling and by the fifth generation of any outbreak , we’d expect 32 new cases to appear; by the tenth there would be 1024 on average.  If R can be reduced to less than 1 the outbreak will end. This is because if each person who is infected in turn infects less than one person, the outbreak will reduce; an R of 0.5 would mean that 10 infected people would infect five others, who would then infect another 2.5.

To put R into context,  R for pandemic flu is 1-2 patients, which is about the same for Ebola. Sars virus in 2003 had an R of 2-3 just like covid-19. Smallpox had an R of 4-6 and chicken pox is slightly higher at 6-8. Yet these numbers are low in comparison to what measles is capable of. In a fully susceptible community, a single measles case can generate more than 20 new infections.

R will also vary across different locales and between regions of the UK. Boris Johnson, said on 10 May that R in England was currently “between 0.5 and 0.9, but potentially just below 1.” In Scotland, first minister Nicola Sturgeon said it was between 0.7 and 1.

In Germany, the Robert Koch Institute reported on 9 May that the R had risen to 1.1 just days after the country began to ease lockdown measures. The same effect was also seen in Denmark when it reopened primary schools in April: the R rose from 0.6 to 0.9.

One of the reasons R has become so popular is that is can be estimated from real-life data.

However, experts have warned that without up-to-date and comprehensive data the reproduction number is a “blunt monitoring tool.”

Only with extensive surveillance and rapid testing of suspect cases in the wider community, in hospitals, and in care homes, and other places at high risk, can we be truly confident that the epidemic is in decline and that it is safe to relax measures.

The Shepwayvox Team

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