Dr. Fauci has said a vaccine could be ready for public distribution in the next year and a half or less, though the estimate may prove too optimistic. Typically vaccine clinical trials take 10 to 15 years, and require a significantly higher safety bar to clear than other drugs, since vaccines are injected into healthy people.

Urgent as the need is, public health leaders warn, moving too quickly could have disastrous consequences not only for reining in COVID-19, but for vaccines more broadly. If a vaccine is released that doesn’t work well or yields dangerous side effects—especially in the face of an historic pandemic—it could empower anti-vaccine activists and reduce support for other longstanding vaccines that have gone through rigorous and exhaustive testing.

“There have been times in the past where vaccines have been justifiably rolled out and they haven’t measured up,” said Dr. William Schaffner, a professor of preventive medicine and infectious diseases at Vanderbilt’s School of Medicine. “And that set vaccinology, vaccine acceptance, and confidence in government way back.”