I don't know, but think it's reasonable to think that part of the reason for the 'W' shaped mortality curves in 1918 was not that the virulence of the 1918 flu

I don't know, but think it's reasonable to think that part of the reason for the 'W' shaped mortality curves in 1918 was not that the virulence of the 1918 flu was higher for people 20-40, but rather the incidence of infection was just much higher for all those soldiers running around in transport ships etc in Europe. Another example where we draw inferences on something phenomena, without taking into account the broader context (in this case incidence rate) https://wwwnc.cdc.gov/eid/article/12/1/05-0979-f2 https://wwwnc.cdc.gov/eid/article/12/1/05-0979-f2