A new paper suggests that 40-60% of the population already has T-cells which are activated to defend against covid, even though they've never actually encountered covid-19, because covid-19 is so similar to other coronaviruses.
To explain this on a fairly technical level which might lose some readers:
T-cells identify viruses by the little doodads sticking out of their bulbous bits, and I think the ideas is that these other coronaviruses have doodads which are not exactly like covid's but which are close enough to covid's that T-cells know to treat it as enemy (and block those doodads so they can't jimmy their way into healthy cells).
"... CD4+ T cell responses were detected in 40-60% of unexposed individuals. This may be reflective of some degree of crossreactive, preexisting immunity to SARSCoV-2 in some, but not all, individuals..."— david friedberg (@friedberg) May 21, 2020
I had previously discarded this hypothesis based on the evidence of extremely high WuFlu prevalence in some prisons, 70%+ but with very high numbers of asymptomatic cases."...but it is tempting to speculate that the crossreactive CD4+ T cells may be of value in protective immunity, based on SARS mouse models "— david friedberg (@friedberg) May 21, 2020
But what if they're infected but fighting it off without much trouble thanks to previous corona-virus infections?
When the facts change . . .