> Transmissibility (or Basic Reproduction Number - R0) is an intrinsic characteristic of a disease.

I'm not talking about the theoretical R0, which assumes an immunologically naive population. I'm talking about the real world reproductive ratio, R, including existing immunity. Estimates of real world R for influenza are are even lower than the 2-4 your source quoted. Eg, this paper estimates a median of just 1.3 for a typical year's seasonal influenza: https://bmcinfectdis.biomedcentral.com/articles/10.1186/1471...

With COVID-19's R0 estimated at 2-3, it could easily be the case that our measures are sufficient to completely wipe out the flu. But not COVID-19: https://www.thelancet.com/journals/laninf/article/PIIS1473-3...

Re: measles, note that pre-vaccine, almost everyone got infected with measles at some point in their life, usually as a child. You actually see this in how effective the early childhood measles vaccination programs were: even though only children were being vaccinated, a small % of the total population, measles rates dropped dramatically(1) almost immediately. The adults had already gotten it and were already immune. So to wipe it out, we just needed to protect those who hadn't been infected: the new, uninfected, children born every year. The measles vaccine is attenuated live measles after all. So by vaccinating, we just get the inevitable infection over with in a controlled way, with the odds rigged _enormously_ in your favor.

1) https://ftp.historyofvaccines.org/index.php/content/graph-us...

> With COVID-19's R0 estimated at 2-3

I haven't been keeping up with it, but that's the lowest estimate I've seen yet. Back when it was still only known to be in China it was estimated at 5-6, but quickly went down to 3-4 for the first several months or so of last year, then some estimates brought it back up to 5-6..