Those that acted early, here is a new and noteworthy paper.
What worked: Early school, church, and theater closures. The estimate for banning public gatherings depends on the test used.
What didn’t work: Closing dance halls, isolating flu cases, banning public funerals, and making influenza notifiable.
Early-acting cities were especially effective at stopping many casualties at the first flu wave, rather than being devastated by later waves as well. Letting down one’s guard too early was a common problem.
The authors stress that this study used only 17 cities with highly correlated explanatory variables. Still, in a case like this some chance at knowledge is better than no knowledge. Here is a related paper. Here is a New York Times discussion of the results.