AstraZeneca and Oxford University's Covid-19 vaccine has similar efficacy against the coronavirus variant currently circulating in Britain and Ireland as it does to the previously circulating variants, the university has said.

The findings, released in a preprint paper and not peer-reviewed, also detailed recent analysis showing that vaccination with the shot results in a reduction in the duration of shedding and viral load, which may translate into a reduced transmission of the disease, Oxford University said.

Meanwhile, British regulators have received extra trial data from AstraZeneca that supports their view that the Covid-19 vaccine developed with Oxford University is effective in the elderly, a vaccines official said.

Britain has been rolling out the shot among all age groups after the Medicines and Healthcare products Regulatory Agency (MHRA) was the first regulator to approve it in December.

A number of European countries, including Ireland, have said more data is needed before it is given to those over 65.

"Since (initial approval) we've seen more data coming through from AstraZeneca as more people are completing the trial, which highlights again that efficacy in the elderly is seen, and there's no evidence of lack of efficacy," said Munir Pirmohamed, Chair of the Commission on Human Medicines' Covid-19 Vaccines Benefit Risk Expert Working Group.

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Separately, the British government has announced that all adults aged 50 and over should receive a coronavirus vaccine by May.

Ministers had previously refused to give a firm date, saying only that the first nine priority groups would be vaccinated by the spring.

Downing Street repeatedly declined to define when spring ends during a briefing for journalists yesterday.

But the Cabinet Office announced this morning that the government intended to vaccinate all nine cohorts by May.

It said the UK's vaccination programme planned to have reached all nine priority cohorts by May, which it said gave the government confidence to commit to holding local elections that month.

According to the government's vaccines delivery plan, some 27 million people in England, and 32 million people across the UK, are estimated to fall in the first nine groups.

The government is on track to vaccinate the first four cohorts by 15 February, including NHS and care home staff, care home residents, and all those aged 70 and over.

So far, more than 10.4 million people in the UK have received their first dose of a coronavirus vaccine.

Categories five to nine include all those aged 50 and over, as well as adults aged 16-65 in an at-risk group.

Prioritisation for the rest of the population has yet to be determined.

Additional reporting PA