Estimating the COVID-19 R number: a bargain with the devil? - The Lancet Infectious Diseases

Type Journal Article Author Chris T. Bauch URL https://www.thelancet.com/journals/laninf/article/PIIS1473-3099(20)30840-9/fulltext Series Comment Publication The Lancet Infectious Diseases Date 22/10/2020 DOI https://doi.org/10.1016/S1473-3099(20)30840-9 Accessed 2020-10-27 17:44:48 Abstract Bob May's limerick alludes to both the promises and dangers of characterising epidemic control by a single number. The basic reproduction number (R0) is the average number of infections produced by a single infectious person in a population with no immunity. R0 has a close relative named the effective reproduction number (R), which is the average number of infections produced by a single infected person in a population with partial immunity. In The Lancet Infectious Diseases, You Li and colleagues2 estimate how the imposition and lifting of non-pharmaceutical interventions (NPIs) changed the R number for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in 131 countries in the first half of 2020. If the R value is less than 1, an epidemic eventually dies out because each infected person generates less than one new infection. Ending an epidemic by keeping the R value below 1 could take a long time if there are currently many infections, like the proverbial small rudder on a big ship. However, when the R value is higher than 1, the epidemic could continue to grow. R can also change over time: NPIs such as closing schools, physical distancing, and mask use can reduce R. Hence, R is often used to gauge whether pandemic mitigation is working. Li and colleagues compared daily estimates of R at the country level against a database describing which NPIs each country applied and when. Generally, they found that imposing NPIs reduced R, and lifting them later on increased R. School closure, a public events ban, requirements to stay at home, and internal movement limits—both when being imposed and when lifted—had the biggest individual effects, changing R between 3% and 25%.