Around four in five older primary school children in most of the UK are now likely to have Covid-19 antibodies, new analysis suggests.
The estimates, which are for eight to 11-year-olds, are as high as 82.0% in both England and Scotland, with 79.7% in Wales.
Northern Ireland has a slightly lower figure of 72.2%.
The presence of coronavirus antibodies suggests someone has had the infection in the past or has been vaccinated.
But only a small number of children aged eight to 11 in the UK outside clinical trials have received any Covid-19 vaccine, meaning most people in this age group with antibodies will have them due to a coronavirus infection.
The figures, which have been calculated by the Office for National Statistics (ONS), are the first of their kind to prioritise a new higher threshold for antibodies, which better reflects the proportion of people likely to have a strong enough antibody response to provide some protection from a new Covid-19 infection.
The higher threshold will also provide the earliest signs of any changes in antibody levels, the ONS said.
The latest data is for the week beginning January 31 and shows how antibody estimates for older primary school children rose sharply during the recent Omicron-driven wave of infections.
In England, the estimate has jumped from 31.8% of eight to 11-year-olds at the start of December to 82.0% by early February. In Scotland it has increased from 30.4% to 82.0%.
Wales has seen a jump from 24.6% to 79.7%, and in Northern Ireland it has risen from 14.4% to 72.2%.
The Joint Committee on Vaccination and Immunisation (JCVI) recommended in December that five to 11-year-olds in clinically vulnerable groups should be offered two doses of Covid-19 vaccine, delivered eight weeks apart.
This rollout had only recently begun in the period covered by the latest antibody estimates.
All five to 11-year-olds will be offered the vaccine later in the spring, the Government announced last week, following new recommendations from the JCVI.
First doses of Covid-19 vaccine have been available for 12 to 15-year-olds in the UK since September last year, with jabs being offered in schools as well as at local vaccination centres.
More than nine in 10 children in this age group are now likely to have coronavirus antibodies, the ONS found.
Estimates range from 95.1% of those aged 12-15 in Scotland to 91.7% in Northern Ireland, with 94.4% in England and 93.4% in Wales.
All ONS estimates are based on a sample of blood test results from people in private households and are subject to uncertainty, given they are based on samples that are part of the wider population.
Antibody positivity is defined by having a fixed concentration of antibodies in the blood.
A negative test result occurs if there are no antibodies, or if antibody levels are too low to reach the threshold at the time of testing.
It takes between two and three weeks after infection – or vaccination – for the human body to make enough antibodies to fight coronavirus.
Levels are expected to decrease over time irrespective of vaccination or natural infection, especially when exposure to the virus is reduced, the ONS said.
This is because our bodies stop making antibodies when they are not needed.
