None of the trials of Covid-19 candidate vaccines currently under way are designed to detect a reduction in serious outcomes such as hospitalisation or death, according to a leading public health expert.
Writing in the BMJ medical journal, associate editor Peter Doshi warned that not even the phase three trials under way in the race for a vaccine can prove their product will prevent people contracting Covid-19.
He said those hoping for a breakthrough to end the pandemic would be disappointed, with some vaccines likely to reduce the risk of Covid-19 infection by only 30%.
"None of the trials currently under way are designed to detect a reduction in any serious outcome such as hospital admissions, use of intensive care, or deaths," he said.
"Nor are the vaccines being studied to determine whether they can interrupt transmission of the virus."
A linked editorial in the BMJ medical journal comments on the release of the Covid-19 vaccine trial protocols.
The World Health Organization has identified 42 candidate vaccines in clinical trials, ten of which are in the most advanced phase three stage.
This is where a vaccine's effectiveness is tested on a large scale, generally tens of thousands of people across several continents.
But Dr Doshi, assistant professor of pharmaceutical health services research at the University of Maryland School of Pharmacy, said that even the most advanced trials are evaluating mild rather than severe disease.
This may be down to the numbers of people involved in trials, he said, pointing out that the majority of confirmed Covid-19 infections involve mild or no symptoms.
And few if any current trials are designed to find out whether there is a benefit among the elderly, a key at-risk constituency.
Without enrolling frail and elderly volunteers in trials in sufficient numbers, Mr Doshi said "there can be little basis for assuming any benefit against hospitalisation or mortality".
He added that children, immunocompromised people and pregnant women had largely been excluded from trials, making it unlikely that the experiments will address key gaps in our understanding of how Covid-19 develops differently among individuals.
Several trials have already been halted after participants became ill.
Many countries plan to prioritise vulnerable people once a vaccine is available, but Dr Doshi said that those hoping for a miracle end to the pandemic would have to wait.
He said that several pharmaceutical firms had designed their studies "to detect a relative risk reduction of at least 30% in participants developing laboratory confirmed Covid-19".
Recent studies have also confirmed that it is possible for someone to be re-infected with Covid-19, a development that may impact how governments' form their vaccination plans.