The authorized vaccines were initially tested in early phase clinical trials, in which scientists evaluated different doses, checked for the expected immune responses and monitored for potential safety issues. To find out whether the shots prevent disease and are safe, the vaccines that passed the first set of tests were then evaluated in phase 3 trials. The Moderna phase 3 trial included about 30,000 volunteers, while the Pfizer/BioNTech and Johnson & Johnson trials included about 44,000 each. In each of these trials, half of the participants received a placebo, while the other half received the vaccine, and the volunteers were followed over time to see if they fell ill with COVID-19 and had any adverse reactions.
The trials were overseen by independent data and safety monitoring boards, and the results were reviewed not only by the Food and Drug Administration, which made the decision to authorize the vaccines for emergency use, but also by an outside panel of experts advising the agency. Another independent group, the CDC’s Advisory Committee on Immunization Practices, also reviewed the data and recommended use of the vaccines.