In some cases, a vaccine that hasn’t been properly tested could make people sicker. And if there are complications, the public relations problems could mount, spurring an anti-vaccination sentiment…

There are several examples of how fast-tracking vaccines can backfire. A massive campaign to vaccinate against the 1976 swine flu flopped when the disease turned out to be mild, but hundreds of people suffered a rare nerve disorder after vaccination. And a vaccine used in some European countries against H1N1 flu in 2009 caused some people to develop the sleep disorder narcolepsy.

The Trump administration has already allowed one vaccine maker to begin testing its coronavirus shot in people before completing standard safety testing in animals — normally the first step in the long process of determining a vaccine’s safety and efficacy.