From the net (two reports from 2008 and 2009 discuss this):
“In summary, just before the 1918 death spike, aspirin was recommended in regimens now known to be potentially toxic and to cause pulmonary edema and may therefore have contributed to overall pandemic mortality and several of its mysteries. Young adult mortality may be explained by willingness to use the new, recommended therapy and the presence of youth in regimented treatment settings (military). The lower mortality of children may be a result of less aspirin use. The major pediatric text of 1918 recommended hydrotherapy for fever, not salicylate....”
Read another article that said that similar rates of death occurred in countries where aspirin was NOT used, “so the debate continues”. But obviously, massive overdosing with aspirin was happening, and no doubt had terrible effects.