I think you are overestimating the spiritual and technological reach of the gedolim you mentioned. World-wide acceptance of the Mishna Brurah and the Aruch hashulchan was attained way after this period. The Chazon Ish was a young man of 39-40 and had not yet attained global recognition. The Spanish flu was unlikely to have been addressed by these gedolim as a psak applicable to all, but rather a local recommendation that remained local, and was not necessarily recorded for posterity.
A better example may be the cholera epidemic of 1849 and the famous psak and actions of Rav Yisroel Salanter. But even that was a local psak and was not binding or meant for all of klal yisrael. The details of that episode are a subject of debate until today, and it only played out on a local level.
On that note, important public controversies that relied on mail and transportation usually played out over a period of years and not weeks or months. Today these things are instantaneous, then they were not.