Know the difference b/w placebo and corona vaccine trial

Placebo is something that is not a medicine that has no side effect. Bharat Biotech has given clarification after Anil Vij was infected after taking the first dose of Corona vaccine. The company said- The clinical trial of covaxine is based on 2 doses, which is given on 28 days. It will be effective 14 days after the second dose. It becomes effective after both doses of covaxine.