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. Then the agency compares whether they received the vaccine or the placebo (a harmless, inactive substance that looks identical to the vaccine being tested). If a greater share of those who are sick received the placebo instead of the vaccine, then the vaccine has met an acceptable efficacy standard.
Learn more about this process here.
Covid-19 vaccines currently authorized in the United States demonstrated extremely high efficacy in clinical trials. The Pfizer and Moderna vaccines had efficacy rates of 95 percent and 94.1 percent, respectively. Therefore, either the Pfizer or Moderna vaccine will provide similar protection against infection from COVID-19. People should accept the first vaccine that becomes available to them in their area.