Oxford University scientists leading the global search for a coronavirus vaccine are to recruit "very healthy" over-55s to help with clinical trials.
The next phase of testing will focus on how older adults' immune systems respond, the Oxford vaccine group said on Friday.
Scientists are looking for 10,260 people from across the UK to take the jab, considered a front-runner in the world race for a vaccine.
The Government has said that, if the treatment proves successful in human trials, up to 30 million doses could be available for the UK by September.
But Professor Andrew Pollard, the head of the Oxford Vaccine Group, said it was "very difficult" to know when scientists will have proof that it is effective.
Work began in January on the vaccine, which uses a virus taken from chimpanzees and has been developed by the University of Oxford's Jenner Institute and the Oxford Vaccine Group.
The first phase of trialling involved 160 healthy volunteers aged between 18 and 55.
Now scientists want to recruit more than 10,000 people across the country for phases two and three, which involve vastly increasing the number of volunteers and expanding the age range to include older adults and children.
Adult participants in the phase two and three groups will be randomised to receive one or two doses of either a vaccine known as ChAdOx1 nCoV-19, or a licensed vaccine (MenACWY) that will be used as a "control" for comparison.
ChAdOx1 nCoV-19 is made from a weakened version of a common cold virus from chimpanzees that has been genetically changed to make it impossible for it to grow in humans.
This has been combined with genes that make proteins from the Covid-19 virus (SARS-CoV-2) which play a key role in the infection pathway of the SARS-CoV-2 virus.
Prof Pollard said the next phase of vaccine testing will focus on the immune response in older adults, adding: "Now we are looking at whether older adults have a similar immune response, then looking at those in the front line.
"There are two groups – the first are those over the age of 55, and they are divided into those being 55 and 70, and those who are over 70. And in that group we are looking very closely at immune responses, particularly in the oldest adults where often immune responses are a bit weaker [than] in younger adults."
He said a second group of 10,000 frontline workers will also participate in the study.
Asked who should come forward to take part, Prof Pollard said "very healthy individuals" will initially be selected, adding that clinical studies were progressing "very well".
Sarah Gilbert, professor of vaccinology at the Jenner Institute, said: "We have had a lot of interest already from people over the age of 55 years who were not eligible to take part in the phase one study, and we will now be able to include older age groups to continue the vaccine assessment.
"We will also be including more study sites in different parts of the country."
Production of the vaccine has already been scaled up ahead of the trial to prepare as early as possible for potential future deployment.
AstraZeneca said this week that it had the capacity to manufacture one billion doses of the University of Oxford's potential Covid-19 vaccine and could begin supply in September.