Chief executive of NHS England Amanda Pritchard said “yesterday was the biggest day yet for Covid booster jabs” with more than 800,000 people having received a top up in the past three days.
Chancellor Rishi Sunak said the data “does not suggest that we should be immediately moving to Plan B”.
However, a jabs expert warned that the vaccination rollout will not be enough to bring current infection rates under control.
Watch: Chancellor ignores questions about Covid plan B
“Well, the Prime Minister actually just said that we’re looking at the data all the time, as you would expect us to,” Mr Sunak told BBC One’s The Andrew Marr Show.
“We’re monitoring everything, but at the moment the data does not suggest that we should be immediately moving to Plan B, but of course we will keep an eye on that and the plans are ready.”
Professor Adam Finn, who is on the Joint Committee of Vaccination and Immunisation (JCVI), warned against complacency in what he said is a “worsening” situation.
He said people need to be testing themselves, wearing masks and avoiding crowds in enclosed spaces in order to prevent “a real meltdown”.
But Downing Street has insisted there is still spare capacity in the NHS and that Plan B will only be activated if it comes under “significant pressure”.
Measures would include working-from-home guidance and the mandatory use of face masks.
Vaccines minister Maggie Throup said Plan A is “working” and “where we need to be”.
Asked where case numbers will have to get to before the public will be asked to work from home, Ms Throup told LBC: “The public has been very patient in doing what we’ve asked them to do.
Yesterday was the biggest day yet for Covid booster jabs: more than 325,000 people getting vital protection. In the past three days over 800,000 people have had their booster jab.
Thank you for coming forward, and if you're eligible, book your jab today. https://t.co/V8A8tCIw9H
— Amanda Pritchard (@AmandaPritchard) October 24, 2021
“And I think Plan A has actually opened up people’s lives and that’s so important because if we do need to take further measures I’m sure they’ll have appreciated exactly the freedoms we’ve been able to offer them at this time.”
The government has launched a media blitz encouraging people to get a booster jab, and is encouraging those not yet vaccinated to do so.
Prof Finn said that while vaccines are very effective at stopping people from getting seriously ill, they are not so effective at stopping infections altogether or stopping the virus from spreading.
His comments came after another prominent adviser to the government on Covid-19 said he is “very fearful” there will be another “lockdown Christmas” as he urged the public to do everything possible to reduce transmission of the virus.
Professor Peter Openshaw, a member of the New and Emerging Respiratory Virus Threats Advisory Group (Nervtag), said case numbers and death rates are currently “unacceptable”.
He said measures such as working from home and mask-wearing are “so important” as part of efforts to control the spread of Covid.
Prof Openshaw also advised people to “take matters into your own hands”, telling BBC Breakfast on Saturday: “Don’t wait necessarily for government policy.”