In an interview this morning, he would also not be drawn on the likelihood of exams being run as planned this year.
However, parents should send their children to primary school as planned tomorrow, the prime minister said, despite increasing calls for all schools to move to remote learning on the return from the Christmas holidays tomorrow.
Speaking on the BBC's The Andrew Marr Show, Mr Johnson said that while he understood people’s anxieties over safety, there was no doubt in his mind that schools are safe.
However, he would not guarantee that all schools would reopen for in-person teaching from 18 January, and said that the government would continue to assess the impact of the tier 4 measures.
Need to know: London U-turn not about school safety concerns, says DfE
The prime minister said: "The evidence is not clear, because we're looking at tier 4 and what happens in tier 4 areas, and we need to see whether those extra steps that we've all taken in tier 4 areas are going to work in driving the virus down.
"We've got to keep things under constant review, but we will be driven not by any political considerations but entirely by the public health question."
And when asked whether or not GCSE and A-level exams would go ahead as planned this year, Mr Johnson gave no clear answer, only predicting that the situation with Covid generally would be better by the spring.
However, when asked if parents should send their children into school tomorrow, he replied: "Yes, absolutely [parents] should in the areas where schools are open. And what we're doing clearly is grappling with a new variant of the virus, which is surging, particularly in London in the South East. And that's why we've had to take exceptional measures for some parts to close primary schools temporarily – not something anybody wants to do."
Coronavirus: The risk to school staff 'is very small'
Mr Johnson added: "We've really fought very hard throughout this pandemic across the country to keep schools open for lots of reasons. Schools are safe. It's very, very important to stress that the threat to the risk to kids, to young people is really very, very, very small. Indeed, as the scientists continually attest to, risk to staff is very small.
"And. of course, the benefit of education is so huge. Overwhelmingly, we want to keep our young people and children in education because that's the best thing for them."
Brighton and Hove City Council has advised its primary schools to delay the return to in-person teaching and teach remotely until 18 January. Meanwhile, Birmingham City Council has written to education secretary Gavin Williamson to urge him to reconsider the start of face-to-face teaching in primary schools for those in tier 4 areas.
Mr Johnson said that councils should be guided by public health advice that schools are safe and that the priority was children's education. He added that the government wanted to work with councils and local authorities.
Safety fears over reopening schools
Four teaching unions and two support staff unions are calling on the government to delay in-person teaching in primary schools and to move all teaching online over concerns about teachers’ safety.
The NEU teaching union has advised members of their “legal right” not to return to classrooms from tomorrow, and the NAHT school leaders' union has joined forces with the Association of School and College Leaders (ASCL) to begin a legal process to force the DfE to release the scientific advice on school Covid safety and to demonstrate that it has given full and proper consideration to the health and safety of pupils and staff.
Writing in the Mail on Sunday, Mr Williamson said that “keeping our kids out of classrooms is damaging” and that the government has been working throughout the holidays to make the return to school “as safe as possible”.
He wrote: “These decisions are not political calculations, they are concrete steps to support our children’s education, futures and dreams – which must not be put on hold.
“The safety of teachers and pupils will always be paramount, but we must all move heaven and earth to get children back to the classroom where they best thrive.”
'We cannot furlough young people's learning"
Ofsted chief inspector Amanda Spielman has today warned that young people’s learning and wider development must not be “furloughed”.
Writing in the The Sunday Telegraph, Ms Spielman said that “we must renew and maintain the consensus that children’s time out of school should be kept to the absolute minimum.”
She wrote: “There is a real consensus that schools should be the last places to close and the first to reopen, and having argued for this since last spring, I welcome it. Because it is increasingly clear that children’s lives can’t just be put on hold while we wait for vaccination programmes to take effect, and for waves of infection to subside. We cannot furlough young people’s learning or their wider development.”
Meanwhile, the children’s commissioner for England, Anne Longfield, told BBC News this morning that teachers should be prioritised for vaccination.
Ms Longfield said that if schools are to be closed for in-person teaching, it should be for a minimum amount of time, and that this time should be used to vaccinate teachers.
She said: “I've argued for months and months now that schools need to be a priority for children not only with education but also their wellbeing and that schools should be the last to close and the first to open, so it's a serious moment for children.
"And if there have to be closures – we're already seeing closures in secondary schools for two weeks – but if there have to be closures at all, it must be the absolute minimum amount of time, and that time must be used very, very well. I would say [prioritise] testing, of course – we know that there are plans in place for that – but also I would like teachers to be offered vaccination as a priority.
"That's something we haven't heard yet from government, but something I think is very necessary."