To prove the safety and effectiveness of a vaccine, researchers recruit large numbers of volunteers at risk of exposure to the targeted disease. In randomized clinical trials participants are divided into two groups, with one receiving the candidate vaccine and the other a placebo. Researchers then compare rates of infection in each group to gauge a vaccine's effectiveness.
Pfizer and BioNTech, who developed a COVID-19 vaccine that the companies say is 95% effective, began clinical trials on July 27 with 43,661 participants, approximately half of whom received the vaccine. On Nov. 18, the companies reported 170 confirmed cases of COVID-19 among trial participants: 162 in the placebo group and eight in the vaccinated group, supporting the reported 95% effectiveness. Among confirmed cases, the companies noted 10 were severe, with nine in the placebo group and one in the vaccinated group.