Will the New Coronavirus Keep Spreading or Not? You Have to Know One Little Number

Whenever there’s a new outbreak, scientists rush to calculate a number called R₀, or R-naught

By Jeffery DelViscio, Amanda Montañez & Tanya Lewis edited by Dean Visser

Passengers wear face masks to protect against the spread of the new coronavirus as they arrive on a flight from Asia at Los Angeles International Airport on January 29, 2020.

Mark Ralston *Getty Images*

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As the novel coronavirus continues to spread widely in China and around the world, scientists are racing to understand how infectious the disease is and to project how many more people could be sickened in the coming weeks or months.

For disease outbreaks, epidemiologists have a term for describing the average number of people an infected individual will spread an illness to in a susceptible population: it is known as the basic reproduction number, or R₀ (pronounced “R-naught”). If the R₀ is less than one, the outbreak will fizzle out. If it is greater than one, the outbreak will continue. Early estimates place the R₀ for the 2019 novel coronavirus (SARS-CoV-2) in the range of two to three. For comparison, the R₀ of SARS (a related coronavirus) was two to four when it caused a deadly outbreak in 2003, and the R₀ of measles is 12 to 18.

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But a reproduction number is not fixed. For example, it can change as people become immune to a pathogen or change their behavior to avoid sick people. The effective reproduction number for the new coronavirus is likely lower than the estimated R₀—but it is still too early to predict how the current outbreak will play out.

Editor’s Note (2/11/20): The video in this story has been revised after posting. It originally incorrectly referred to SARS as “sudden acute respiratory syndrome.” The full name is “severe acute respiratory syndrome.”

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