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 hotspots in Africa and Latin America, to get convincing results.
"Ironically, if we're really successful using public health measures to stamp out the hot spots of viral infection, it will be harder to test the vaccine," said Francis Collins, director of the National Institutes of Health in the United States.
A vaccine is seen as essential to ending a pandemic that has killed nearly 370,000 people and infected more than 6 million so far, with world leaders looking at inoculation as the only real way to restart their stalled economies.
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.
Vaccine trials work by randomly dividing people into a treatment group and a control group, with the treatment group getting the experimental trial vaccine and the control group getting a placebo.
All participants go back into the community where the disease is circulating, and subsequent rates of infection are compared. The hope is that infections within the control group will be higher, showing the trial vaccine is protecting the other group.
Among the first COVID-19 vaccines to move into phase two, or mid-stage, trials is one from the U.S. biotech company Moderna and another being developed by scientists at Oxford University supported by AstraZeneca. The United States in July is planning to launch vast efficacy trials of 20,000 to 30,000 volunteers per vaccine.