The Oxford coronavirus vaccine has shown a strong immune response in adults in their 60s and 70s, raising hopes that it can protect age groups most at risk of illness from the virus.

Researchers say the Lancet phase two findings, based on 560 healthy adult volunteers, are "encouraging".

They are also testing whether the vaccine stops people developing Covid-19 in larger, phase three trials.

Early results from this crucial stage are expected in the coming weeks.

Three vaccines - Pfizer-BioNTech, Sputnik and Moderna - have already reported good preliminary data from phase three trials, with one suggesting 94% of over-65s could be protected from Covid-19.

The challenge with developing a Covid vaccine is to trigger the body to fight back against the virus no matter how old someone is.

Older people's weaker immune systems mean vaccines do not tend to function as well as they do in younger people.

These trial results from the University of Oxford, peer-reviewed in the Lancet, suggest that may not be a problem.

They show that older adults aged 56-69 and over 70 had a similar immune response to younger adults aged 18-55.

'Protect most vulnerable'

Dr Maheshi Ramasamy, an investigator at the Oxford Vaccine Group, said: "We were pleased to see that our vaccine was not only well tolerated in older adults, but also stimulated similar immune responses to those seen in younger volunteers.

"The next step will be to see if this translates into protection from the disease itself."

Two weeks after the second dose, more than 99% of participants had neutralising antibody responses. These included people of all ages.

The T-cell response - another measure of how well the immune system responds - peaked two weeks after the first dose of the vaccine, regardless of age.

"The robust antibody and T-cell responses seen in older people in our study are encouraging," Dr Ramasamy said.

"The populations at greatest risk of serious Covid-19 disease include people with existing health conditions and older adults.

"We hope that this means our vaccine will help to protect some of the most vulnerable people in society, but further research will be needed before we can be sure."

Older adults were also less likely to experience side-effects, which were usually mild.

And there were no serious safety issues relating to the vaccine, called ChAdOx1 nCov-2019.

Volunteers in the trial were put into groups and given one or two doses of the vaccine or a dummy jab. The reaction of their immune systems was assessed on the day they got the jab, then one, two and four weeks after both doses.