> The impact of that 2009 pandemic was considerably blunted by human cross reactive T-cell immunity in the older population who had seen the long-lost relative

> The impact of that 2009 pandemic was considerably blunted by human cross reactive T-cell immunity in the older population who had seen the long-lost relative of H1 protein back before 1957. That 50 year old "pre-vaccination" did not wane and was cross reactive after 50 years of mutation and adaptation in pigs, and was still effective enough to protect against severe disease and death. Wait, didn't the 57's H1N1 strain reappeared unchanged in the 70's and didn't a vaccination campaign happen then ? (I seem to remember it's mostly likely an unintentional lab leak, which would explain it didn't change in 25 years).