Other, more advanced polio vaccines designed to supplement the original Salk vaccine have also failed, causing oral polio vaccine viruses to revert to wild-type and become virulent again. New clinical models, such as human challenge trials, can pick up where clinical trials leave off and enable the rapid development of prognostic efficacy data for many infectious diseases. HOW HUMAN CHALLENGE TRIALS CAN ACCELERATE VACCINE DEVELOPMENTThe most common bottlenecks in vaccine development are cost, risk, safety and time. Fortunately, clinical models such as human challenge trials provide an opportunity to move past these bottlenecks. Given these risks, the industry approached the COVID-19 vaccine development process with caution, much as it has for countless development projects in the past.