evidence: The second dose of a COVID-19 vaccine induces a powerful boost to a part of the immune system that provides broad antiviral protection, according to a study led by investigators at the Stanford University School of Medicine. [...] "The second shot has powerful beneficial effects that far exceed those of the first shot," Pulendran said. "It stimulated a manifold increase in antibody levels, a terrific T-cell response that was absent after the first shot alone, and a strikingly enhanced innate immune response." Unexpectedly, Pulendran said, the vaccine — particularly the second dose — caused the massive mobilization of a newly discovered group of first-responder cells that are normally scarce and quiescent.