England's chief scientific adviser today warned Britain was still in a 'very uncertain phase' of the pandemic, despite Covid cases falling for four days in a row.
He suggested that the Government's winter Plan B - which includes compulsory masks, working from home and vaccine passports - should not be taken off the table yet.
Scientific advisers have told the Government that the epidemic could shrink naturally next month without curbs because of a combination of the booster vaccine rollout and growing natural immunity in children.
But warning against complacency today, Sir Patrick told the BBC Radio 4 Today programme: 'You need to absolutely be prepared (for plan B) and as soon as you start thinking "am I, or am I not going to do this? It looks close" is the time you need to push beyond your natural reluctance to do it and do it.
'This is obviously something the Government will have to consider carefully but we need to be ready to move fast if that occurs.'
Sir Patrick warned last month that the Prime Minister should go 'harder and faster' than appears to be necessary to contain the virus.
Medics and trade unions have called for Britain to switch to 'Plan B' immediately, which will see the reimposition of face masks and work from home guidance.
But ministers have so far held their nerve, insisting the NHS is not under 'unsustainable' pressure. Sources say there is a 'less than 20 per cent chance' that the plans will be activated.
Sir Patrick Vallance said more restrictions should be imposed at the first sign of trouble
LONDON SCHOOL OF HYGIENE AND TROPICAL MEDICINE: These charts show the impact of returning to normal level of social mixing in three months (bottom) versus remaining cautious for a year - and the impact this would have on infections (left), admissions (middle) and deaths (right). The models show cases plummeting by November in both scenarios thanks to natural immunity but rising in spring (bottom) when vaccine protection is expected to wane
Releasing all Covid restrictions on 'Freedom Day' may have actually helped save hundreds of lives despite outcry at the time, a study led by 'Professor Lockdown' Neil Ferguson has found.
Imperial College London researchers praised the roadmap more generally, saying that it was 'largely successful at limiting infection levels'.
They said No10 timed the easing of restrictions well because the dates of each step of the roadmap allowed vaccines to get into the arms of those most at risk.
And the study said it was prudent to delay 'Freedom Day' nearly a month from its original date on June 26 after the emergence of the Delta variant.
This decision alone prevented at least 2,000 hospital admissions per day, they found. This ultimately saved countless lives.
Experts previously criticised No10 for being 'unscientific ' and argued Boris Johnson lifted restrictions too early on July 19.
But scientists like Professor Christ Whitty, England's chief medical officer, said