Thousands of young people have flocked to Twickenham Stadium in London for their vaccinations as the UK races to contain the fast-spreading Indian variant.
Up to 15,000 first doses of the Pfizer-BioNTech Covid-19 vaccine were initially made available to those aged 30 and over in south-west and north-west London.
By Monday afternoon, NHS officials decided to allow anyone aged 18 and over to receive an injection, leading to what was described as "Disney-like queues" outside the stadium, home of the England rugby team.
The one-off event, held at the UK’s largest inoculation centre, was likely the largest-scale vaccination of young adults so far in England.
It diverged from the government’s staggered distribution policy, under which vaccines are available only to those aged 30 and older in most circumstances.
The scenes at Twickenham came amid growing calls for the fourth and final stage of England’s road map out of lockdown on June 21 to be delayed to stop the spread of the variant first detected in India.
The variant is more transmissible than the UK’s previously dominant strain, but vaccines appear to be effective against it.
However, scientists say more people need to be inoculated before plans to lift social distancing rules and other restrictions can go ahead.
The government said it would announce on June 14 whether the relaxation would be delayed.
On Monday, 3,383 new cases were confirmed in the UK – the sixth consecutive day that the figure has topped 3,000, while one death was reported. The majority of new cases related to the Indian variant.
Prof Ravi Gupta, a government adviser, said there was a "real risk now of generalised transmission in young people who are not vaccinated”.
He suggested a one-month delay until schools are closed for holidays for the next stage to proceed.
“We've got to a really good position and the easing has been well done so far,” he told Sky News on Tuesday.
"But we've obviously got this complicating factor which is this new virus that was identified in India … which has a new set of properties that we did not anticipate happening. We really should be making sure we think about what we're doing in the context of this new, unknown virus."
He said “we are not too far from reaching the sort of levels of vaccination that would help us contain the virus”.
“If you look at the costs and benefits of getting it wrong, I think it is heavily in favour of delay,” he said.
About 75 per cent of British adults have received their first dose of a Covid-19 vaccine and more than 48 per cent have both doses.
Prof Adam Finn, another government adviser, said the country remained vulnerable despite the success of the vaccination drive.
"The idea that somehow the job is done is wrong – we've still got a lot of people out there who have neither had this virus infection nor yet been immunised and that's why we're in a vulnerable position right now," he told the BBC's Radio 4 Today programme.
Prof Robert Dingwall from Nottingham Trent University disagreed, suggesting "there will be high levels of mild infections in the community for periods of time".
“A handful of people may be seriously ill, even fewer may die," he said.
“But that’s what happens with respiratory viruses and we’ve lived with 30-odd respiratory viruses for since forever.”
He said the unvaccinated younger age groups were "intrinsically at much lower risk" from serious illness.
"Many of the scientists who’ve been talking over the weekend simply haven’t adjusted their expectations to understand that – (for these people) Covid is a mild illness in the community," he said.
Small Business Minister Paul Scully said Prime Minister Boris Johnson would make a decision in the next few weeks.
“Clearly, we know the fact that case numbers are going up, but we want to make sure we act on data,” he said.
“This isn’t fudge, this is making sure we don’t speculate by using really good info to make good decisions.”
Meanwhile, leading global bodies and UK MPs urged rich countries to donate Covid-19 vaccines to poorer countries to close the “dangerous gap” over access to the shots.
The heads of the World Health Organisation, International Monetary Fund, World Bank Group and World Trade Organisation said the lack of vaccines in developing nations made it easier for new variants of the virus to take hold.
They said the shortfall was creating a “two-track” pandemic, with low-income nations receiving “less than 1 per cent of vaccines administered so far”.