However, it's also possible they just met R. R-naught is the basic reproduction number for an illness, which is to say that it represents the average number of infections you would expect to be the result of one infected person. If R-naught for a condition is two, then each person infected will likely infect two additional people on average. This is a concrete number that is characteristic of an infectious disease, and it is not affected by the development of treatments or preventions. It's just intrinsic to the condition. R, on the other hand... is the variable that's used to describe a disease's effective reproduction rate. This number takes into account that some people are immune to conditions, maybe because they're vaccinated, so a proportion of the population won't end up getting infected even if they're exposed to a sick person.