Chickenpox, on the other hand, is way more contagious, Gangavarapu says. Chickenpox has an R0 of about nine or 10. So each person with chickenpox infects about 10 other people on average. Outbreaks are explosive.

For SARS-CoV-2, the R0 has actually risen over the course of the pandemic as the virus evolved. When the coronavirus first emerged in 2019, SARS-CoV-2 was slightly more contagious than flu, Gangavarapu says. “The initial COVID-19 strain had an R0 between two and three.”

Then about a year later, the virus began to mutate quickly. The alpha variant emerged, likely in the U.K., and was more transmissible than the original strain. A few months later, the delta variant emerged, most likely in India. It was even more transmissible than alpha.

“For the delta variant, the R0 is now calculated at between six and seven,” Wenseleers says. So it’s two- to three-times as contagious as the original version of SARS-CoV-2 (R0 = 2 to 3) but less contagious than the chickenpox (R0 = 9 to 10).