Vaccines typically go through a lengthy FDA approval process that requires copious amounts of completed phase 3 trial data and can take many months. However, during a public emergency, the FDA can issue an Emergency Use Authorization (EUA) to allow use of an unapproved vaccine while trials are ongoing as long as the vaccine meets certain criteria (including the benefits outweighing the risks) and no alternatives exist. The FDA laid out the criteria for Covid-19 vaccines in October, including an efficacy of at least 50% and safety data for at least two months after complete vaccination.
EUAs have existed since the 1930s, but it wasn’t until 2004, following the 9/11 attacks and concerns about bioterrorism, that vaccines could receive them. Using EUAs for Covid-19 vaccines has critics, but the FDA still requires safety and effectiveness data from tens of thousands of trial participants before granting an EUA. Most importantly, the FDA still requires the VRBPAC — composed of independent experts who are not FDA employees — to review the vaccine data before deciding on an EUA.