The pause of the trials came after reports that the United States was aiming for fast-track authorization or approval of a vaccine before November's presidential election.
Leading U.S. and European vaccine developers have pledged to uphold scientific safety and efficacy standards for their experimental vaccines and not bow to political pressures to rush the process.
AstraZeneca has already agreed to supply close to three billion doses to governments across the globe - more than any other vaccine project.
The WHO's chief scientist said the pause in the trials should serve as a "wake-up" call that there would be ups and downs in the development of a vaccine.
"Inevitably with such a large trial there will be times when safety issues arise," said Peter Openshaw, professor of experimental medicine at Imperial College London.
"We must all hope that there are no future events and that the vaccine proves both safe and effective."
