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.