The statement came in Prime Minister's Questions in the House of Commons as Mr Johnson attempted deflect MPs anger at the rising number of deaths during the Covid-19 pandemic.
It was reported this week the number of deaths have passed the 100,000 mark and many MPs are demanding that the PM publishes a route map out of lockdown during coming weeks.
Mr Johnson delivered the grim news for millions of children and parents saying he realised "how frustrated" they will be.
"We will not persist for a day longer than necessary but nor will we relax too soon. The overall picture should become clearer by mid-February. Reopening schools will be our first priority."
The PM said that fast progress was being made on vaccinations with 6.8 million people - 13 per cent of the UK population - being given doses so far.
He said the NHS is on track to hit its goal of covering the four most vulnerable groups by February 15. The jabs should give them full protection three weeks after receiving them.
Mr Johnson said: "We hope it will be therefore safe to begin the reopening of schools from Monday, March 8." The Prime Minister said there is not enough data as yet to assess the full impact of the vaccine programme, saying “what we do know is that we remain in a perilous situation”.
He added that he hoped to be able to reveal his plan for easing lockdown to MPs from February 22.
The March 8 date has been suggested as it is three weeks after the date – 15 February – that the government has pledged to have vaccinated all of the top four priority groups.
A decision on schools should be shared two weeks before they are reopened, according to an earlier statement from education secretary Gavin Williamson.
In an interview with Sky News last week, health secretary Matt Hancock wouldn't guarantee that schools would reopen before Easter, and yesterday Anne Longfield, the Children’s Commissioner for England, said there was a "real danger" schools could remain shut until summer.