The Jenner Institute’s vaccine effort is not the only one showing promise. Two American companies, Moderna and Inovio, have started small clinical trials with technologies involving modified or otherwise manipulated genetic material. They are seeking both to demonstrate their safety and to learn more about dosing and other variables. Neither technology has ever produced a licensed drug or been manufactured at scale.

A Chinese company, CanSino, has also started clinical trials in China using a technology similar to the Oxford institute’s, using a strain of the same respiratory virus that is found in humans, not chimps. But demonstrating the effectiveness of a vaccine in China may be difficult because Covid-19 infections there have plummeted.

Armed with safety data from their human trials of similar vaccines for Ebola, MERS and malaria, though, the scientists at Oxford’s institute persuaded British regulators to allow unusually accelerated trials while the epidemic is still hot around them.

The institute last week began a Phase I clinical trial involving 1,100 people. Crucially, next month it will begin a combined Phase II and Phase III trial involving another 5,000. Unlike any other vaccine project now underway, that trial is designed to prove effectiveness as well as safety.

The scientists would declare victory if as many as a dozen participants who are given a placebo become sick with Covid-19 compared with only one or two who receive the inoculation. “Then we have a party and tell the world,” Professor Hill said. Everyone who had received only the placebo would also be vaccinated immediately.

If too few participants are infected in Britain, the institute is planning other trials where the coronavirus may still be spreading, possibly in Africa or India.

“We’ll have to chase the epidemic,” Professor Hill said. “If it is still raging in certain states, it is not inconceivable we end up testing in the United States in November.”

Carl Zimmer contributed reporting.