The first wave of the Covid-19 pandemic may be waning. For vaccine developers, that could be a problem.
Scientists in Europe and the US say the relative success of lockdown and physical distancing policies in some countries means virus transmission rates may be at such low levels that there is not enough disease circulating to truly test potential vaccines.
“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 US.
A vaccine is seen as essential to ending a pandemic that has killed nearly 370 000 people and infected more than 6 million, with world leaders looking at inoculation as the only real way to restart their 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.
“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 re-purposing at Britain’s Warwick Business School.
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.
With Covid-19 outbreaks in Britain, Europe and the US coming down from their peak and transmission rates of the virus dropping, a key task for scientists is to chase fluctuating outbreaks and seek volunteers in sections of populations or in countries where the disease is still rife.
Among the first Covid-19 vaccines to move into phase two, or mid-stage, trials is one from the US bio-tech company Moderna, and another being developed by scientists at Oxford University supported by AstraZeneca. In July the US plans to launch vast efficacy trials of 20 000 to 30 000 volunteers per vaccine.
Collins said US health officials would tap government and industry clinical trial networks in the country first and use mapping to detect where the virus is most active. They will also consider looking abroad if domestic disease rates fall too far, he said.
The US government has experience in Africa of testing vaccines against HIV, malaria and TB.
Adrian Hill, director of the Jenner Institute at Britain’s Oxford University which has teamed up with AstraZeneca, started mid-stage trials last month, which he said would aim to recruit around 10 000 Britons.
He said that with Covid-19 disease transmission rates dropping in the UK, there was a possibility that the trial would have to be halted if they didn’t have enough infections to yield a result.
“That would be disappointing, and at the moment it’s unlikely, but it’s certainly a possibility,” Hill added.