With covid19 many patients are able to continue their lives and infect others without being obviously sick themselves. This limited the ability to contain outbreaks without extensive testing.
* R0 when the disease appears first time and people dont know about it
* R0 when people learned about disease and changed behavior.
Assuming disease is new, one person sick with SARS will infect more people then one person sick with COVID.
Due to stronger symptoms and shorter time to symptoms, we learn SARS is issue, change our behavior and quickly close small area. SARS will spread less. In covid, by the time we figure area is infected, a lot of people are infected elsewhere. It takes us more time to react.