Access to vaccines wasn't an issue with the Spanish flu since flu vaccines didn't exist yet. I think it's more than social conditions, though. This virus does seem to mutate at a prodigious rate even compared to the highly mutable influenza viruses. That mutability then combines with the social issues to enable new variants to spread. If all countries had Canada's rate of vaccination, it would be far more difficult for mutations to develop and spread. And global mobility, of course, is what got us here. Were we still in a world where it took days rather than hours to cross between continents, the spread would be far slower and the disease more localized.