The longer the infectious period of a disease, the more likely a person who has it can transmit the disease to other people.
Presumably, R0 is affected by vaccination, then if R0 comes down and testing continues then a bit later we see positivity rate come down, then active cases will start to follow after that.
The following factors are taken into account to calculate the R 0 of a disease: Infectious period Some diseases are contagious for longer periods than others.