#SARS-CoV2 vaccines : Slow is fast | Science Advances
Clinical trials proceed in three phases. In Phase I and II the vaccine is tested on a small cohort of individuals to determine if the vaccine is safe in the short term and elicits immune responses. A Phase III trial follows, which tests the vaccine in a large cohort for actual efficacy against infection and potential adverse effects. Phase III trials are necessarily time-consuming and require that infection among the control group is sufficiently high to draw a conclusion.
Many advocate “fast-tracking” these trials, and some wish to rely soley on evidence of induction of neutralizing responses. However, this could be catastrophic. In 1966, a large trial of a vaccine for Respiratory Syncytial Virus (RSV) found that the immunized cohort actually faired significantly worse upon infection. There is some reason to worry that the same may occur with some SARS-CoV2 vaccines.