Experts have warned that Omicron's increased transmissibility means it is “coming at us like an express train”.
Asked whether shielding would return for the millions of clinically extremely vulnerable people in England, the spokesperson for the prime minister said: "We will keep... things under review, based on the latest clinical advice for those that are clinically extremely vulnerable.
"Not only do we have vaccinations available, we also have brand new, cutting edge therapeutics in the form of monoclonal antibodies and antivirals which are being rolled out across our NHS, particularly targeted at those most in need, those most vulnerable, which provide an extra layer of protection for them."
The government have repeatedly stressed that prioritising clinically extremely vulnerable groups and older people in the booster rollout is part of their plan to protect those most at risk from the virus.
The comments come after the government were told to "get a grip" by Labour over unclear advice to shielders as the Omicron variant of COVID-19 surges across the UK.
“The prime minister has lost his authority and the public are paying the price," Shadow minister for disabled people Vicky Foxcroft told Yahoo News UK.
“Many clinically extremely vulnerable people have struggled to get appointments with already overworked clinicians, so they can’t determine their risks, this is on top of a lack of clear guidance about whether they must now once again shield.
"Ministers need to get grip and ensure clinically extremely vulnerable people are assured and supported.”
On Wednesday, chief medical officer Professor Chris Whitty said he wanted to avoid the return of shielding due to its negative health impacts in other areas.
“Shielding came with some very serious downsides for the people involved in the shielding, including loneliness and mental health issues, practical problems," he said.
“It’s not an area we want to go down if there’s any way of avoiding doing so.”
While shielding officially came to an end on 19 July, 22% of clinically extremely vulnerable people continued to shield - living a life dramatically different to the rest of the country.
The prime minister said on Monday afternoon, following a lengthy cabinet meeting, that a lockdown and tighter restrictions would not be introduced before Christmas.
However, he also said the government would not hesitate to take further action, adding that they were monitoring the data "hour by hour".
He also told the public: "Please exercise caution as you go about your lives."
Minesh Patel, Head of Policy at Macmillan Cancer Support, said: “For many people with illnesses that affect their immune system such as cancer, the lockdown that began way back in March 2020 has never truly stopped.
“It is vital that the government continues to be clear in its communication and guidance for those people who are in the clinically extremely vulnerable group. Many are already feeling the effects of being isolated from their loved ones for so long."
The UK recorded 91,743, a rise on yesterday's figures and the second highest number of COVID-19 cases ever recorded in the UK.
Watch: COVID-19: Boris Johnson 'reserves possibility' of further restrictions and says govt is following the data 'hour by hour'