Notice the two peaks. If all the assumptions of the SIR model held for this outbreak, Theorem 2.1 would imply there would be only one peak. Also, the US experienced one peak every year except during the pandemic years 1918, 1958, 1968, 2009, when it experienced two or more waves. What is the mechanism(s) that is generating the multiple peaks? Nobody knows.
Recently in [10], we used relatively simple extensions of the SIR model to exhibit five plausible mechanisms, each of which could have generated the two peaks during 2009, both quantitatively and qualitatively.