“We don’t have much experience with these types of vaccines,” says Tom Frieden, former director of the Centers for Disease Control and Prevention. Adenovirus vectored vaccines, like Oxford’s, are relatively new, and mRNA vaccines—like those developed by Moderna and Pfizer—have never been used before in humans. The Trump administration is reportedly making plans to distribute the Moderna and Pfizer candidates, which are still in Phase III trials, in the U.S. by early November.

Kinch is worried that many countries are placing big bets on these newer vaccine technologies, while shunning old, established ones like inactivated-virus vaccines, which China is pursuing. That’s one more reason to take safety with Covid-19 vaccines very, very seriously.

“Well designed and executed studies are so crucial,” he wrote in an email. “Autoimmune sensitives can be rare … and often take time (weeks or months) to develop.” Which means rushing a trial can be deadly. “A one in a thousand [reaction] sounds rare except when you scale those numbers up to 350 million Americans, or seven billion humans worldwide, the outcomes can be disastrous.”