Vaccines that don’t demonstrate overall safety in small phase 1 and 2 trials will not go any further. Some of the leading coronavirus candidates have the advantage of being based on familiar, time-tested formulas, including candidates containing a viral protein or inactivated whole virus, with or without an immune-boosting adjuvant. Other contenders represent newer technologies, including those based on RNA, DNA or virus-like particles, but even some of these approaches have already led to licensed vaccines against other diseases or at least gone through prior human trials. And they have a few advantages of their own, such as fast manufacture or smaller dose requirements.
Vaccine development normally takes years, starting with preclinical laboratory work. But most pandemic vaccines are being tested in overlapping trial phases to compress the timeline. In Phases 1 and 2, safety and early immune responses are assessed. In Phase 3, researchers will also look at vaccine efficacy: does it protect, who does it protect, and how well?
Whether a pandemic vaccine requires one dose or two, and how far apart shots must be given, can impact how practical it will be to distribute and how much it costs. Phase 3 trials will also reveal a vaccine’s efficacy: how much of the population it is likely to protect, and to what degree.
Experts project the leading candidates might protect roughly 70% of vaccinated people against infection or against severe COVID-19 if they are infected. The U.S. FDA has set a minimum threshold of 50% efficacy to even consider a vaccine for approval. Still not clear is how long that protection will last, but it could be as little as a year, AstraZeneca CEO Pascal Soriot said last June and BioNTech chief executive Ugur Sahin echoed in November.