There were unprecedented levels of coronavirus infection in England this month with Omicron almost completely replacing Delta, data suggests.
The highest prevalence was seen in primary school children at 7.81%, while overall it was 4.41% between January 5 and 20 this year.
It means that during this period one in 23 people in England was infected with Covid.
This is the highest figure since the beginning of Imperial College London's React-1 study, which has been running since May 2020.
It is also more than three times higher than the prevalence in the previous round of the study, which looked at November 23 to December 14.
Overall, the data suggests the peak of the wave was around January 5, with cases flattening off by the middle of the month.
The data also indicates that, while prevalence was decreasing among adults in January, it was rising in children.
Meanwhile, of the 3,582 people who said whether or not they have previously had Covid, two thirds (64.6%) reported a past infection.
Researchers urged caution, however, saying it is unknown when those past infection were and that PCR tests can return positive results for more than 90 days after diagnosis.
The increase in prevalence corresponds with Omicron becoming the dominant variant in England, the researchers added.
Over the study period, prevalence increased in every region compared to the previous report, with the highest figure recorded in the North East at 6.85%, followed by Yorkshire and the Humber at 5.58%.
Professor Paul Elliott, director of the React programme from Imperial's School of Public Health, said: "There is good news in our data in that infections had been rapidly dropping during January, but they are still extremely high and may have recently stalled at a very high prevalence.
"Of particular concern is that there is rapidly increasing prevalence among children now they are mixing more following the start of the school term and, compared with December, prevalence in older people aged 65-plus has increased seven- to 12-fold, which may lead to increased hospitalisations.
"It's therefore vital that we continue to monitor the situation closely to understand the impact of the Omicron variant, which now makes up almost all infections in the country."
Health Secretary Sajid Javid said: "It's reassuring to see Covid-19 infections beginning to slow across the country as we move back to Plan A.
"Covid-19 rates are still high so, as we learn to live with the virus, it is vital we continue to be vigilant - wash your hands, let in fresh air, get tested and, if you haven't already, get boosted now."
