The Spanish flu pandemic ended primarily because the virus had infected so many people globally that there were not enough susceptible hosts left for it to continue spreading effectively. This led to a form of herd immunity where enough individuals had developed immunity either by surviving the infection or by dying from it, which caused the virus's spread to gradually diminish. There was no dramatic declaration of the pandemic's end; by mid-1920, the flu had largely faded, although some places experienced later, milder waves. The end was also influenced by seasonal factors and possibly the virus mutating to a less lethal form, but the main driver was the widespread immunity achieved in the population. Unlike modern pandemics, no vaccine ended it, as vaccines were not developed in time to impact this pandemic significantly.
