Just days ago, scientists leading the University of Oxford's coronavirus vaccine development expressed optimism about their progress — more than 1,000 people in the United Kingdom have been inoculated already, and 10,000 more will be given the vaccine in May and June. But there's some cause for concern, The Telegraph reports.Professor Adrian Hill, director of the University's Jenner Institute, said what was formerly an 80 percent chance of developing an effective vaccine by September — possibly in time for a potential second wave of infections — has dwindled to 50 percent. That's not because the team no longer believes in its work, which is reportedly still going well. Instead, the U.K.'s infection rate decline may make it tough to gauge the vaccine's efficacy. "It's a race against the virus disappearing, and against time," Hill said.Hill only expects fewer than 50 of the 10,000 trial volunteers to catch the virus, which has faded since the U.K. and other countries implemented strict lockdowns, and if it turns out that fewer than 20 test positive, the study's results may be useless.The vaccine showed promise when it was tested in six rhesus macaque monkeys earlier this year, but it will obviously need to show that it provides the same protection in humans before it can be distributed.More stories from theweek.com Trump keeps falsely accusing Joe Scarborough of murder, and it's long past weird Trump denies wanting to move the Republican National Convention to his Doral resort, citing ballroom size Brazil's Bolsonaro accepts little blame for his country's COVID-19 disaster, but he wears a face mask