Oxford University said that "in large trials such as this, it is expected that some participants will become unwell and every case must be carefully evaluated".
Charlotte Summers, lecturer in intensive care medicine at Cambridge University, said the pause showed the researchers' commitment "to putting safety at the heart of their development program".
"To tackle the global Covid-19 pandemic, we need to develop vaccines and therapies that people feel comfortable using, therefore it is vital to maintaining public trust that we stick to the evidence and do not draw conclusions before information is available," she said.
That public trust will be crucial to convincing a public that is impatient for a vaccine — and in some corners skeptical.
Among the impatient is US President Donald Trump, who has been accused by rival Joe Biden of "undermining public confidence" by regularly raising the possibility that a vaccine will be ready before November's election.
The Republican president is under pressure as the US toll continues to rise, nearing 6.5 million cases on Saturday with more than 193,000 deaths — by far the most in either measure in the world.
Biden also called Trump "reckless" for holding a rally in the Nevada city of Reno even after the venue had to be changed because the event breached local Covid-19 restrictions.