The reproductive rate or R0 is an estimate of the number of people an infected person can infect, on average, at any given point in time.
An R0 of 18, for example, means an infected person can infect 18 others. SARS-CoV-2 has an R0 of about two, meaning one infected person can infect two others.
Scientists calculate the R0 to gauge how fast an infection can spread. Decisions about whether to impose or relax lockdowns are influenced by the R0: if it falls below one, lockdowns are eased; if it increases, regulations are tightened. The R0 is usually presented as a range. For COVID-19 – in the absence of any mitigating (containment) measures – the R0 is said to be in the range of two to three people, which is lower than the range of 12-18 for a similar airborne infection like measles, but higher than for influenza, which is 1.2 to 1.3.
TIP: Find out which population the R0 refers to. The R0 is derived from mathematical modeling based on many assumptions, and so can vary greatly. For example, in an elderly care institution that has a cluster of COVID-19 patients, the R0 could be far higher than in the general community which includes all age groups.
Epidemiology Infectious Disease Transmission