To determine whether a disease has a potential to become an epidemic or pandemic, scientist use a mathematical term R0 (pronounced “R- naught”). It is a basic reproduction number, which shows how many healthy people can get infected by one person who has a virus. For example, when an infected person who come into close contact with a group of people passes the virus to two of them, the R0 = 2. Based on this number scientists can tell epidemic potential of the infectious disease.
R0 < 1 – each infected person can infect on average less than one healthy individual. The disease is declining in this case
R0 = 1 – each infected person can pass the virus to one another person on average. The disease is in stable state, but it won’t cause an outbreak.
R0 > 1 – each person infects more than one healthy individual. The disease spreads fast and has a potential to cause epidemics.
Coronavirus in the beginning was often compared to seasonal influenza to assess its severity, because it is one of the viruses we experience or regular basis. Influenza outbreaks happen every year and despite vaccination many people still die of it every year. Average R0 for seasonal influenza is 1.5
For the Novel Coronavirus, WHO estimated R0 to be between 1.4 and 2.5 (information available as of March). However, multiple studies done by other researches show that R0 could be up 6.49. The number is still calculated and adjusted and can differ for different countries.
There’s a number of factors taken into account when calculation R0.