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A London doctor has warned the government against 'overpromising and under-delivering' on the Covid booster rollout.
Dr Ammara Hughes said getting jabs into people's arms was different this time because fewer volunteers were available to help.
On Tuesday, Boris Johnson said adults would be offered a booster to increase protection against the Omicron variant by the end of January.
"I think we shouldn't overpromise and under-deliver frankly. I have yet to see the workforce that will deliver those jabs," said Dr Ammara Hughes.
"Last time we had a mass run on vaccinations we were in lockdown and had a lot of people furloughed so we had a big volunteer workforce.
"But those people have now gone back to work and their jobs. "So the crucial thing will be how many people can we actually deploy?" she added.
Most new England cases of the Covid Omicron variant have been found in London, according to latest official figures.
Nine further cases were confirmed in England on Wednesday bringing the total in England to 22. Fourteen of those were identified in London boroughs.
The breakdown of cases by local authority in London is:
The breakdown of cases elsewhere in England is:
North Norfolk: 1
South Cambridgeshire: 1
Three Rivers: 1
(Source: UK Health Security Agency)
The Health Secretary Sajid Javid said getting an offer of a Covid booster jab to all adults by the end of January will be a "huge ask".
But Mr Javid said he was confident the NHS would be able to cope.
"It is a huge ask. It’s something I do believe that people who work in the NHS, all the volunteers, are up to it," he told Sky News.
"But it is a huge ask and that is why I am calling it a national mission. This has become, from a health point of view, the most important thing we can do to protect ourselves, protect the progress that we have made."
Dr Jenny Harries, UKHSA chief executive, added: “It is very likely that we will find more cases over the coming days, as we are seeing in other countries globally and as we increase case detection through focused contact tracing.
“That’s why it’s critical that anyone with Covid-19 symptoms isolates and gets a PCR test immediately.”