Dr Jenny Harries, England's deputy chief medical officer, has warned that a new coronavirus wave could easily take off again, despite cases and deaths falling.
Dr Harries told a No 10 news conference that infections were now below 100 per 100,000 of population in every part of the country and were back to where they were in September.
But she issued a warning and said: "This is a level at which a new wave could easily take off from again."
She said that deaths were falling faster than other indicators, suggesting the vaccine was having an impact, but that there was still "substantial strain" on the NHS.
Earlier in the press conference the Prime Minister Boris Johnson said that thanks to the "huge national effort to keep kids at home" the spread of coronavirus had been reduced significantly.
Mr Johnson said: "So, today we've been able to take that crucial first step on what we hope is our cautious but irreversible road map to freedom."
Mr Johnson said more than a third of the UK population have now received a coronavirus vaccine.
But he warned the number of patients being admitted to hospital with Covid-19 each day is eight times higher than "the lows of last summer" and that it is "more vital than ever" to follow the rules.
Boris Johnson praised parents and teachers in the conference and said it marked a "big day and an emotional day" for millions of families across England.
He said: "We all know that the education of our children is so important that the greater risk now is keeping them out of school for a day longer.
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"I want to thank all the teachers who have got their schools ready and who have been teaching throughout the period - whether that is remotely or in person. Your work has been astonishing."
Mr Johnson also thanked parents who have been teaching their children at home and said: "We all know that the burden has disproportionately fallen on women - often holding down jobs and providing childcare at the same time."
