What is R and does it matter?




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Mathematicians and public-health experts watched through their fingers in May as British Prime Minister Boris Johnson unveiled a series of charts to explain how the government would guide Britain out of coronavirus lockdown. Perhaps most prominent was a colourful dial with a needle hovering near a single digit: 1.

The dial indicated R, a now-totemic figure in the COVID-19 pandemic. The nation, said Johnson, would set a COVID-19 alert level, to be “primarily determined” by the number of coronavirus cases, and by R, the reproduction number.

To infectious-disease experts, Johnson’s focus on the reproduction number as a guiding light for policy was worryingly myopic. They worry about placing too much weight on R, the average number of people each person with a disease goes on to infect.

In this pandemic, R has leapt from the pages of academic journals into regular discussions by politicians and newspapers, framed as a number that will shape everyone’s lives. As Germany’s chancellor, Angela Merkel, explained in a widely viewed video this April, an R above one means an outbreak is growing, and below one means that it is shrinking. In many countries, it is publicly reported every week. In June, epidemiologists at the Harvard T.H. Chan School of Public Health in Boston, Massachusetts, announced a website where anyone can look up the value for any country — and for many smaller regions — in the world.

But fascination might have turned into unhealthy political and media fixation, say disease experts. R is an imprecise estimate that rests on assumptions, says Jeremy Rossman, a virologist at the University of Kent, UK. It doesn’t capture the current status of an epidemic and can spike up and down when case numbers are low. It is also an average for a population and therefore can hide local variation. Too much attention to it could obscure the importance of other measures, such as trends in numbers of new infections, deaths and hospital admissions, and cohort surveys to see how many people in a population currently have the disease, or have already had it

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