At the end of April, crunching a process that normally takes about five years into less than four months, Gilbert and her colleagues at Oxford’s Jenner Institute started a human trial on 1,100 people. When Gilbert testified before a parliamentary committee in early July, one member compared her effort to going into a shed and coming out with a jet engine. Gilbert’s team has leapfrogged other vaccine contenders to the point where it will likely finish vaccinating subjects in its big 10,000-person efficacy trial before other candidates even start testing on that scale, Kate Bingham, chair of the U.K. government’s Vaccine Taskforce, told the parliamentary committee in early July. “She’s well ahead of the world,” Bingham said. “It’s the most advanced vaccine anywhere.”

Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases (NIAID), has sounded a note of caution about Oxford’s front-runner status. “You’ve got to be careful if you’re temporarily leading the way vs. having a vaccine that’s actually going to work,” he told the BBC recently. Most vaccines in development fail to get licensed. Unlike drugs to treat diseases, vaccines are given to healthy people to prevent illness, which means regulators set a high bar for approval and usually want to see years’ worth of safety data. In the Covid-19 pandemic, it’s not yet clear what regulators will accept as proof of a successful and safe vaccine. The U.S. Food and Drug Administration has said a vaccine would need to be 50% more effective than a placebo to be approved and would need to show more evidence than blood tests indicating an immune response. Regulators in other countries haven’t spelled out what would be acceptable.

Gilbert has voiced remarkable confidence in her chances, saying the Oxford vaccine has an 80% probability of being effective in stopping people who are exposed to the novel coronavirus from developing Covid-19. She has said she could know by September. Asked by MPs in early July whether the world would have to struggle through the winter without a vaccine, Gilbert said, “I hope we can improve on those timelines and come to your rescue.”