Plus the transmission rate,lack of vaccine.
Contagion documentary with Hannah Fry was on BBC 4 last night .
They modelled exactly this scenario .
A transmission rate about 0.5 lower than Cono
Unchecked 43 M infections @2% mortality circa 900k.
They ran a model with AFAIR a limited number of the population either immune or vaccinated ,that slowed the spread by about an extra month and brought the total case's down to 30M.
Pretty sure like our Boff's they've used Spanish Flu as the basis for the modelling.
Don't know how good the historical info about Spanish is really ,record keeping etc
Seems to suggest only 25% caught it @2% mortality.
Edit:- it came in three waves.