To measure how well a vaccine works in a clinical trial, the Food and Drug Administration (FDA) first looks at the number of people who fell ill with the disease.
Although rare, some people have reported allergic reactions to the vaccines. An extraordinarily tiny fraction of vaccine recipients have experienced anaphylaxis -- a severe allergic reaction that can impair people's breathing.
People with certain underlying or pre-existing conditions, including cancer, diabetes, kidney disease, obesity, and heart disease, are more likely to experience severe complications from Covid-19 -- so they're strongly encouraged to get the vaccine.
Patient safety is central to the research and development of a vaccine. Like all new medicines or treatments, vaccines must pass a rigorous set of tests before being approved by independent, scientific experts at the U.S. Food and Drug Administration. 
By readying the immune system for future health battles, vaccines protect individuals around the world from dangerous diseases.