The requirement for secondary school pupils to wear face coverings could be reintroduced in Southwark if a request from the borough's director of public health is accepted by national officials.
Sangeeta Leahy took over as Southwark Council's director of public health just a couple of months ago.
At Wednesday's meeting of the health & wellbeing board she revealed that she would be invoking a special procedure which allows directors of public health to seek permission from Public Health England to reintroduce the use of face coverings in secondary schools.
Masks have not been required in English secondary schools since 17 May.
The health & wellbeing board heard that there had been a "recent rapid increase" in the number of COVID-19 cases in the borough, including among school-age young people.
Ms Leahy said: "There are two stories really: one is that case rates are increasing quite rapidly, and the other is that the vaccination programme is also running at pace.
"So both things are going hand-in-hand and it's really important that we keep on top of the outbreaks, as well as accelerating the vaccination programme."
She added: "I agree that our children are seeing more infections, and that is what we would expect, as they are not yet the vaccinated population."
Ms Leahy urged people who have had just a single vaccine dose to remain "vigilant" in their behaviour.
She said: "The national picture is that there are very, very few people who have had both vaccinations who are ending up in hospital.
"The big risk is for those who've only had one vaccination. I think there is a general attitude where people rightly are jubilant about that and feel that they can start to reduce some of the preventative behaviours.
"People are still hugely at risk after just one vaccination. We just can't stress that enough.
"So it's really important not only to go for the second vaccination, but to carry on doing everything you were doing before.
"So that will include potentially meeting outside – if you possibly can – avoid doing mixing with lots of different people, following all of the regulations, and then 'plus some' I would say at the moment, if we are seeing an increase in cases."
She added: "Generally, the hospital picture is encouraging at the moment because it does seem to indicate that between case rates and the vaccination programme we have seen a break in hospital admissions."
Ms Leahy's decision to seek national permission to reintroduce the use of face coverings in secondary schools was welcomed by council leader Kieron Williams and cabinet member for children, young people and education Jasmine Ali.
Cllr Williams said: "The numbers at the moment are still low in the borough and much lower than they've been at many other points in the pandemic.
"The picture in Southwark, across London and nationally is clearly a very worrying one.
"The message that we've all been giving is to please make sure that you are doing everything you can to protect yourself from a virus which is again out there and again has community transmission across the borough.
"And that does mean trying to do your socialising outside, especially while we have got this wonderful weather, and definitely making sure that you get your vaccine and your second dose."
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