Unfortunately R0 is a bit of a vague thing. For seasonal flu, you could think of it as being very low, because most people already have immunity. It's already

Unfortunately R0 is a bit of a vague thing. For seasonal flu, you could think of it as being very low, because most people already have immunity. It's already endemic, so there isn't room for exponential growth. For a new flu (like the 2008 swine flu) it is extremely high, which is why people barely did anything to contain it back then -- it was just impossible. What we have right now is in between.