Phase 3 clinical trials are where a vaccine is tested against the virus in the real world. Pfizer and BioNTech enrolled 43,538 participants around the world and randomly sorted them into two groups: one that received the vaccine and one that received the placebo. Then the company waited to see how many confirmed Covid-19 infections emerged.
The findings reported on Monday showed that 94 volunteers in this group developed Covid-19, but the vast majority of these cases were in the placebo group, indicating that the vaccine seems to be highly effective against the virus. The companies, however, did not disclose the exact split of infections between the treatment and placebo groups.
The more effective a vaccine, the earlier a signal will emerge in a clinical trial. But 94 cases is not enough to conclude the trial. Pfizer and BioNTech said that the endpoint of their trial is when they can confirm 164 Covid-19 infections in their pool.
Therefore, the results are not enough for the Food and Drug Administration to grant full approval to the vaccine. The safety standard for a vaccine is much higher than it is for a treatment, because a vaccine is distributed to vastly more people than a treatment, including those who are healthy and those that may have underlying health concerns. So even rare complications become magnified, making it critical to have a high bar for safety.