Scientists in Europe and the United States say the relative success of draconian lockdown and social distancing policies in some areas and countries means virus transmission rates may be at such low levels that there is not enough disease circulating to truly test potential vaccines.

They may need to look further afield, to pandemic hotspots in Africa and Latin America, to get convincing results…

But running large-scale clinical trials of potential vaccines against a completely new disease at speed is complex, scientists say. Showing efficacy in those trials during a fluctuating pandemic adds extra difficulty – and doing so when outbreaks are waning makes it harder still.

“For this to work, people need to have a risk of infection in the community. If the virus has been temporarily cleared out, then the exercise is futile,” said Ayfer Ali, an expert in drug repurposing at Britain’s Warwick Business School.