- The COVID-19 epidemic slows down before infecting 20% of the population, possibly due to overdispersion and variable susceptibility.
- Around 50% of unexposed individuals have T-cell reactivity to SARS-CoV-2, likely from past coronavirus infections causing common colds.
- Cell-mediated immunity via T cells targets infected cells, unlike antibodies which neutralize virus particles directly.
- Variable infectivity explains why some infect nobody while others cause mass spreading events, affecting herd immunity thresholds.
- Cross-reactive T cells from other coronaviruses may provide partial protection, but its real-world impact is still uncertain.