The first wave of the COVID-19 pandemic may be waning. For vaccine developers, that could be a problem.
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 hot spots in Africa and Latin America, to get convincing results.