No vaccine is 100% effective, especially if the intended target mutates into something deadlier. As I understand it, once a virus evolves, vaccines for the original strain are more or less useless against the new mutation.
Evidently, the Spanish flu virus mutated to become much more virulent over a short period of time in 1918, so any vaccine that could have been produced at the onset of that bug would have had very little effect stemming the eventual pandemic that killed millions.
Evidently, the Spanish flu virus mutated to become much more virulent over a short period of time in 1918, so any vaccine that could have been produced at the onset of that bug would have had very little effect stemming the eventual pandemic that killed millions.