I was aware of this story yesterday but didn't pursue it. So extra thanks to the reader who nudged me out of my torpor and sent me to this report in the Annals of Internal Medicine: Meta-Analysis: Convalescent Blood Products for Spanish Influenza Pneumonia: A Future H5N1 Treatment?. Excerpt:

Studies from the Spanish influenza era reported that transfusion of influenza-convalescent human blood products reduced mortality in patients with influenza complicated by pneumonia. Treatments for H5N1 influenza are unsatisfactory, and convalescent human plasma containing H5N1 antibodies could be an effective therapy during outbreaks and pandemics.

It's just a conjecture, but also a reminder that we need to learn more about (and from) the last great pandemic.