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R0 (R-nought)

For infectious diseases, R0 (R-nought) is the unimpeded replication rate of the disease pathogen in a naive (not immune) population.  An R0 of 2 means that each person with the disease infects two others.  Some things to keep in mind:   

  • An R0 of one means that the disease will spread, but the total number affected will remain constant as new victims exactly balance out those who recover or die.
  • The empirical reproduction rate will change over time as conditions change – more people become immune, social distancing measures make it more difficult to propagate – but the R0 refers to the original base rate.
  • The R0 for Coronavirus is not definitively known, but appears to be quite high (above 2), accounting for its rapid spread.
  • R0 for influenza typically lies in the range below 1.5.
  • Due to the fact that infections expand exponentially (in the literal mathematical meaning, not the popular meaning of “fast”), the speed and scope of a disease’s spread is quite sensitive to changes in the assumed R0.


R0 can be viewed as an intrinsic property of the virus, whereas the Reff takes into account the effect of implemented control measures. The Conversation, CC BY-ND.