It was actually the perverse effects on viral selection created by World War I. The relatively mild first wave turned into a deadly second wave because soldiers

It was actually the perverse effects on viral selection created by World War I. The relatively mild first wave turned into a deadly second wave because soldiers who became mildly ill stayed put in the trenches, while those who had severe cases were put on trains and sent to crowded field hospitals, where they spread the more lethal variants. If they simply suppress symptoms without reducing transmission, the current vaccines being administered may have a similar perverse evolutionary effect (although at least they're not actively selecting for worse strains).