BBC - Future - The places that escaped the Spanish flu
It’s not clear why those attempts to delay the arrival of the disease reduced the mortality rates in these places. But research has suggested that over time, as the virus burned its way through populations, it accumulated mutations that naturally reduced its capacity to cause disease.
Another possibility could be that some populations may have acquired a degree of immunity against the pandemic strain from comparatively harmless seasonal flu strains that were circulating in the years running up to 1918.
While the idea is still debated, it has provided some clues that could help health officials in the fight against future pandemics. Today some countries offer annual vaccinations against seasonal flu strains that can help their populations build up temporary immunity. According to research by Jodie McVernon, an immunologist at the University of Melbourne, this could “provide important protection in the early stages of a new pandemic”.
“The more times you get vaccinated, the more you are exposed to different versions of the flu virus,” adds Markel.