The 1918 Influenza pandemic was notable in that it was particularly devastating to the young and healthy as opposed to the elderly and very young being the usual groups who died from influenza. This means you take out the most productive members of society as well as those in their years of maximum fertility affecting population regrowth. Of course the elderly and very young were also killed by this flu. The Black Death killed up to 1/3 of the population of Europe. This killed feudalism as a social system, opened up Europe for various invasions (Mongols, Ottomans, etc) in the wake of the plague. The emotional impact on the population was huge. So let us assume that the Spanish Flu takes out 1/3 of the populations that are affected by it. OTL the only populations that avoided the flu were those that were isolated by geography or quarantine. An example of the former were villages in Alaska that had so little contact with outside villages, for the latter American Samoa was strictly quarantined and missed the epidemic. Here you'll see isolated populations surviving pretty much intact - however the vast majority of these will be very backward. In 1918 losing 1/3 of the population of the industrialized world, plus a big chunk of places like China and India will be devastating but would not lead to the sort of collapse we would see today with the interconnectedness for food, trade, etc.
Countries like the USA and Russia, which have or potentially have the ability to be relatively self sufficient in food and raw materials would do better in keeping their standard of living at a decent level. Because of the large number of young men in many countries in 1918 being in the military in conditions more amenable to disease transmission, you could very well end up with a large number of "excess" women. Multiple marriage may become socially acceptable. While the populations of Africa and some parts elsewhere that are pretty isolated will maintain themselves better, unless the industrial countries totally collapse as they are too far behind politically and economically.