Schools that ended the requirement for students to wear face coverings this month, in line with government guidelines, are reinstating it again due to the upsurge in Covid cases.

The government canceled the requirement for masks in secondary school classrooms in England on January 20 and since January 27 they are no longer mandatory in common indoor spaces either. But a number of schools that have complied with the rule change are having to reintroduce them a week later due to outbreaks of infections.

In some areas, including Enfield in north London, Calderdale in West Yorkshire and Hertfordshire, public health teams are now recommending that masks be temporarily reinstated in schools where there are significant outbreaks.

At least nine education authorities are also advising that masks remain in place in common areas of schools, despite changing national guidelines. In south-west London, a number of high school students told parents on Friday that a rise in cases meant some age groups would have to start wearing face coverings indoors again and take daily flow tests lateral.

Official figures released by the Department for Education last week show the absence of Covid-related pupils in England jumped by 100,000 in two weeks.

A total of 415,000 children, or just over 5% of the public school population, were absent on January 20, against 3.9% on January 6. More than three quarters of the absent students had tested positive for Covid.

In nearly a quarter of public schools, more than 15% of teachers and leaders were off work. A total of 9% of principals and teachers – 47,000 – were absent on January 20, compared to 44,000 two weeks earlier. A similar proportion of teaching assistants and other staff were also out of school.

Kevin Courtney, joint secretary general of the National Education Union, said: “It is very likely that we will see an increase in disruption in schools over the next few weeks, with an increase in cases among pupils in primary and secondary schools. secondary. The government acted prematurely in removing face masks and acted late in providing ventilation solutions. »

The government has been accused of being slow to provide ventilation solutions. Photograph: Toby Melville/Reuters

The decision on masks in schools comes as new research highlights how reinfections are fueling the pandemic.

Prior to Omicron, protection against symptomatic reinfection for previous variants – such as Delta – was over 90%. But that figure fell to 56% with Omicron, meaning those who suffered once before are much more likely to suffer again, according to a study by researchers at the Weill Cornell Medicine-Qatar research center.

Scientists have been trying to establish to what extent the Omicron wave was driven by reinfections and on Monday the government’s Covid dashboard will start publishing data on reinfections in the UK.

“We know that reinfections are much more common with Omicron,” said Paul Hunter, professor of health protection at the University of East Anglia. “But there are a number of problems – are they less serious? The balance of evidence is that they are less serious than primary infections.

New research by scientists from the UK Health Security Agency conducted during the Alpha wave showed that those re-infected were much less likely to die, with 61% fewer deaths within 28 days of testing positive.

Last week, the React study published its latest report which included data that nearly two-thirds of the 3,582 people infected with Omicron said they had already had Covid. The Imperial College London and Ipsos Mori researchers warned that not all of these infections were necessarily reinfections – some may be people who had received a positive PCR test in his study and are still recovering from a recent infection.

Deciding what counts as a reinfection is part of the problem with attempts to measure it. When the Covid dashboard begins to include reinfection data, repeat positive cases will only be counted separately as a reinfection if there is a gap of at least 90 days. This means that an unknown proportion of people who were infected during the third wave this winter with Delta and then later contracted Omicron will not be included in the new infections.

“Normally reinfections aren’t a problem, but it’s becoming a problem because reinfections are happening a little more frequently with Omicron than before,” Hunter said.

“So when we have the reinfection data, will that change what we understand about the direction of travel of this epidemic? Will adding reinfections make it look, in fact, that it hasn’t leveled off and is not coming back down, but is still rising rapidly?

“Or will it mean that in fact a lot of these reinfections happened at the peak and they are going down even faster than we thought?”