Mass Covid-19 testing of staff and students in England's schools is to pause, following a government recommendation, amid concerns that new, fast-spreading variants of the virus may render daily checks ineffective.
NHS Test and Trace and Public Health England said the emergence of variants with "higher rates of transmission" means the effectiveness of contact testing in schools is "unclear".
The two bodies said the new variant "increases the risk of transmission everywhere, including in school settings".
It means mass daily lateral flow tests should no longer be used as a replacement for self-isolation for coronavirus close contacts.
The government's recommendation to pause daily tests comes just five weeks after Education Secretary Gavin Williamson hailed them as a "milestone moment" in keeping schools open.
In a joint statement, NHS Test and Trace and PHE said: "In light of this changing situation, we now recommend that the rollout of daily contact testing within schools is paused, other than for schools involved in further evaluation."
Following the change in advice, the government agreed to suspend the scheme in "all but a small number of secondary schools and colleges" where it will continue alongside "detailed evaluation".
Explaining the move, a government spokesperson said: "In light of the higher prevalence and rates of transmission of the new variant, further evaluation work is required to make sure it is achieving its aim of breaking chains of transmission and reducing cases of the virus in the community.
“We are therefore pausing daily contact testing in all but a small number of secondary schools and colleges, where it will continue alongside detailed evaluation."
Schools should continue to test staff "twice-weekly where possible", says PHE, pupils should be tested "twice upon return to school" in a bid to identify those who have coronavirus but do not have symptoms.
The Department for Education's (DfE) plan to test college and secondary school pupils and staff daily if they came into contact with cases of Covid-19 to improve attendance had been called into question by experts.
Under the scheme, pupils and staff who were in close contact with someone who has tested positive would be tested for seven days and they would be allowed to remain in school if the test was negative.
It comes after Dougal Hargreaves, the DfE's deputy chief scientific adviser, said on Tuesday that the use of rapid lateral flow devices daily in schools carried a "hypothetical risk of increasing transmission".
He said: "If you are in a very high prevalence area, people are very nervous about the idea that people who have been in contact with an infected case are still attending school whether that is staff or pupils."
Geoff Barton, general secretary of the Association of School and College Leaders (ASCL), said: "We are relieved that lateral flow tests have now been paused as an alternative to self-isolation for individuals who have been in close contact with someone who has coronavirus.
"This use of these tests never really made sense because they don't detect all those with the infection, so we could potentially have ended up with more infectious."
There is no change to the rollout of rapid lateral flow tests in schools and colleges, which is "already proving beneficial in finding teachers and students with coronavirus who do not have symptoms", but tests should not replace self-isolation.
The government's spokesperson added: "Daily contact testing, used as an alternative to up to a whole class having to isolate if a positive case is detected, continues to have the potential to be a valuable tool to keep more young people and staff at school, the best place for students’ development and wellbeing."
"We will continue pilots to gather further data and to build the evidence base for the programme."
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