Certainty of success: three critical parameters in coronavirus vaccine development.

Vaccines for 17 viral pathogens have been licensed for use in humans. Previously, two critical biological parameters of the pathogen and the host-pathogen interaction- and -were proposed to account for much of the past successes in vaccine development, and to be useful in estimating the "certainty of success" of developing an effective vaccine for viral pathogens for which a vaccine currently does not exist. In considering the "certainty of success" in development of human coronavirus vaccines, particularly SARS-CoV-2, a third, related critical parameter is proposed-, at an individual-level, and , at a population-level. Reducing the (and , at a population-level) is predicted to lengthen the incubation period, which in turn is predicted to reduce the severity of illness, and increase the opportunity for an upon exposure to the circulating virus. Similarly, successfully implementing individual- and population-based behaviors that reduce the and , respectively, while testing and deploying COVID-19 vaccines is predicted to increase the "certainty of success" of demonstrating vaccine efficacy and controlling SARS-CoV-2 infection, disease, death, and the pandemic itself.