We know that reinfection is possible with the seasonal flu and one could surmise that it was also possible with the 1918 flu strain. But if reinfection is possible and immunity to the flu doesn't last forever, why did the 1918 pandemic ever end? Shouldn't we have kept seeing massive flu casualties every single year starting in 1918?

Question is inspired by the recent verified COVID reinfections, which is claimed to be the reason why herd immunity is impossible to achieve with the 2020 pandemic.

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https://www.washingtonpost.com/history/2020/09/01/1918-flu-pandemic-end/ explains the 1918 flu strain became weaker and similar to typical seasonal flu:

Over time, those who contracted the virus developed an immunity to the novel strand of influenza, and life returned to normal by the early 1920s, according to historians and medical experts. Reports at the time suggest the virus became less lethal as the pandemic carried on in waves.

But the strand of the flu didn’t just disappear. The influenza virus continuously mutated, passing through humans, pigs and other mammals. The pandemic-level virus morphed into just another seasonal flu. Descendants of the 1918 H1N1 virus make up the influenza viruses we’re fighting today.

“The 1918 flu is still with us, in that sense,” said Ann Reid, the executive director of the National Center for Science Education who successfully sequenced the genetic makeup of the 1918 influenza virus in the 1990s. “It never went away.”

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