R0 is not a biological constant

$R_0$ (Basic reproduction number) cannot be a biological constant. It is affected by biological, sociological, behavioral, and environmental factors. Although there is a value to know what would be the (rough) estimated transmission rate when everything is “normal”.

Let’s consider an extreme example. In a society A, everyone is meeting 1,000 randomly chosen members of A every day. In society B, everyone is living in a huge (1-D) ring and they can only meet the two neighbors on this ring. Now introduce an extremely contagious disease that infects pretty much everyone in contact. In A, anyone who got the disease can infect 1,000 people every day (if A is infinite). Therefore, one can estimate $R_0$ to be 1,000 (or close to 1,000). By contrast, in B, even if the disease is extremely contagious, one can only pass the disease to two others. After it began to spread, the disease can only be passed to another neighbor. Therefore, $R_0 = 1$. It is simply impossible to have a larger value in this configuration.