At least six in ten adults in the UK are now likely to have Covid-19 antibodies, new figures suggest.
The estimates range from 59.2% of adults in Scotland to 69.3% in England, with 63.2% for Wales and 63.5% for Northern Ireland.
The presence of Covid-19 antibodies implies someone has had the infection in the past or has been vaccinated.
It takes between two and three weeks after infection or vaccination for the body to make enough antibodies to fight the virus.
Antibodies then remain in the blood at low levels, although these levels can decline over time to the point that tests can no longer detect them.
The latest estimates are from the Office for National Statistics (ONS) and are based on a sample of blood test results for the week beginning April 19.
They reflect the ongoing impact of the vaccine rollout across the UK, in particular the increasing number of people who have received both doses and are now fully vaccinated.
Government figures show that by April 19, 33,032,120 first doses of vaccine had been given – the equivalent of 62.7% of adults – along with 10,425,790 second doses, which equates to 19.8% of adults.
The ONS said that across all four nations there is a clear pattern between vaccination and testing positive for Covid-19 antibodies – but the detection of antibodies alone is not a precise measure of the immunity protection given by vaccination.
Once infected or vaccinated, the length of time antibodies remain at detectable levels in the blood is not fully known.
It is also not yet known how having detectable antibodies, now or at some time in the past, affects the chance of getting Covid-19 again.
The ONS estimates are for people in private households and do not include settings such as hospitals and care homes.
In England, the highest percentage of adults testing positive for Covid-19 antibodies in the week beginning April 19 was estimated to be the 80 and over age group at 92.3%, followed by people aged 75 to 79 at 88.8% and 70 to 74 at 86.4%.
The lowest percentage was for 25 to 34-year-olds at 46.2%.
In Wales, the highest proportion of adults likely to have tested positive for antibodies was the 80 and over age group at 90.4% followed by 75 to 79 at 87.0%, while in Scotland the highest percentage was for people 80 and over at 88.2% followed by 65 to 69-year-olds at 82.4%.
In Northern Ireland, the ONS uses different age groups due to small sample sizes and estimates 83.4% of people aged 70 and over were likely to have tested positive for antibodies in the week beginning April 19.