Vaccine experts are warning the federal government against rushing out a coronavirus vaccine before testing has shown it's both safe and effective. Decades of history show why they're right.

FDA signals vaccine could green light early
Their concern that the FDA may be moving too quickly heightened when FDA Commissioner Dr. Steven Hahn told the Financial Times that his agency could consider an emergency use authorization (EUA) for a Covid-19 vaccine before late stage clinical trials are complete if the data show strong enough evidence it would protect people.

The commissioner has the authority to allow unapproved medical products to be used in an emergency when there are no adequate or approved alternatives. An EUA is not the same as full approval and it can be withdrawn.
That's what happened with hydroxychloroquine and chloroquine. The FDA granted an EUA to the drugs -- much praised by President Donald Trump -- on March 28. It subsequently revoked its EUA in June after studies showed they were not effective and could also potentially cause serious heart problems.