The R-0 of a virus is also known as the Reproductive Rate. This number is the number of people that a person with the illness infects. With an R-0 of <1, illness is considered to be under control or decreasing. With an R-0 of >1, the illness is spreading. The following table is a comparison of common viruses with known R-0 to the limited information we have of COVID-19. As of March 16, 2020, the WHO estimates an R-0 of between 2.2 and 6.48. The CDC confirmed that there are 2 different strains of the virus. Once the strains have separate tests, we will be able to determine a more precise R-0 number for each one.
Maybe Dr. Mears (Kate Winslet) explains it better:
Check out a more detailed mathematical modeling here:
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