I have already talked about the mounting evidence supporting widespread T-cell immunity against SARS-CoV-2 (Covid-19).
This study on the British Medical Journal (Covid-19: Do many people have pre-existing immunity?) is interesting because it’s one of the few places where a similar situation from 2009 is recalled. In fact, in 2009 we experienced a casedemic with H1N1, that is, a growing number of cases which however did not lead to an actual high mortality rate. I quote:
Swine flu déjà vu
In late 2009, months after the World Health Organization declared the H1N1 “swine flu” virus to be a global pandemic, Alessandro Sette was part of a team working to explain why the so called “novel” virus did not seem to be causing more severe infections than seasonal flu.12
Their answer was pre-existing immunological responses in the adult population: B cells and, in particular, T cells, which “are known to blunt disease severity.”12 Other studies came to the same conclusion: people with pre-existing reactive T cells had less severe H1N1 disease.1314 In addition, a study carried out during the 2009 outbreak by the US Centers for Disease Control and Prevention reported that 33% of people over 60 years old had cross reactive antibodies to the 2009 H1N1 virus, leading the CDC to conclude that “some degree of pre-existing immunity” to the new H1N1 strains existed, especially among adults over age 60.15
The data forced a change in views at WHO and CDC, from an assumption before 2009 that most people “will have no immunity to the pandemic virus”16 to one that acknowledged that “the vulnerability of a population to a pandemic virus is related in part to the level of pre-existing immunity to the virus.”17 But by 2020 it seems that lesson had been forgotten.