Oxford University said on Friday that it was recruiting thousands of volunteers for the next phase in human trials with a coronavirus vaccine, which it said was "going very well."
Up to 10,260 adults and children are enrolled as this extends the age range of those receiving the sample vaccine and includes a number of partner institutions across the UK.
The university in the south of central England started its first studies in April, in which more than 1,000 vaccinations were carried out.
"Clinical trials are progressing very well," said Andrew Pollard, head of the Oxford Vaccine Group, in a statement.
"We are now launching studies to assess how well the vaccine triggers immune responses in older adults and to test whether it can provide protection to the wider population."
However, Pollard told BBC Radio that it was "not possible to predict" when the potential vaccine could be ready for the general population.
"Knowing exactly when we will have evidence that the vaccine will work is a very difficult question," he said.
"Eight attempts are underway"
Much depended on enough people being exposed to COVID-19 in the next phase of the process, he added.
The university and pharmaceutical company AstraZeneca have signed a contract that could make up to 30 million out of 100 million doses available to the UK market by September.
Oxford's efforts include the multidisciplinary vaccine group founded in 1994 to investigate new and improved vaccines, and the Jenner Institute, which deals with both human and animal diseases.
It is based on a chimpanzee adenovirus – a cold virus – that has been genetically engineered to stop replication of COVID-19 in humans.
The first phase of the trial included 160 healthy volunteers between the ages of 18 and 55.
The next phase of the study will include older adults and children between the ages of five and twelve.
A third series of studies will then evaluate how the vaccine works "on a large number of people over the age of 18".
According to the World Health Organization, which has a total of 118 different projects in progress, the university's potential vaccine is one of only eight worldwide that have started trials.
The UK government, which has financed the company with around £ 85m ($ 104m, $ 95m), has identified it as one of the "pioneers" in the global vaccine search.
AstraZeneca said Thursday it had received more than $ 1.0 billion from the United States to fund the production of the vaccine.
(Except for the headline, this story was not edited by NDTV staff and published from a syndicated feed.)