About of half of the UK population in private households is estimated to have Covid-19 antibodies, meaning they have been vaccinated or were previously infected.
A survey by the Office for National Statistics suggests that an estimated 54.9% of people in England had the antibodies in the week ending March 28. This figure is largely unchanged from the previous two weeks, when the last survey was done.
The figures are for people living in their own homes, and excludes those in hospitals and care homes.
The ONS, which conducts the fortnightly Covid-19 Infection Survey, said the proportion of those testing positive for antibodies has levelled off in England, Wales and Scotland.
In Wales, the latest estimate is 49.1% and for Scotland it is 46.0%.
In Northern Ireland, 54.5% of people were likely to have antibodies against the coronavirus – up from 50% in the previous week.
Meanwhile, people have been urged “not to go wild” after social restrictions were eased as one expert warned “we could go back to square one”.
Professor Anthony Harnden, deputy chairman of the Joint Committee on Vaccination and Immunisation (JCVI) said people should not behave in the same way as they did before the pandemic as he called for people to act cautiously for “a little bit more time”.
His words echo those of prime minister Boris Johnson who urged the public to be "exercise restraint" as the shackles finally come off.
Prof Harnden told the BBC: “I think he’s probably concerned, as I am, about the scenes in London that we saw of people actually enjoying the outside, pubs and then the crowded spaces – well, of course what that will do is push infection rates up."
The latest ONS figures also suggest a fall in the proportion of older people with antibodies, which is likely because the data does not yet show the impact of second Covid vaccine doses.
In England, the highest percentage of people testing positive for antibodies is estimated to be the 65 to 69 age group (84.5%), followed by 70 to 74 (82.4%), 75 to 79 (79.8%) and 80 and over (77.6%).
The age group with the lowest percentage was 16 to 24-year-olds (38.7%).
In Wales, the highest proportion of people likely to have tested positive for Covid-19 antibodies was the 65 to 69 age group (79.7%) followed by 70 to 74 (79.2%) and 75 to 79 (75.6%).
In Scotland the highest percentage was again estimated to be among 65 to 69-year-olds (82.9%), followed by 70 to 74-year-olds (78.0%) and 75 to 79-year-olds (69.4%).
In Northern Ireland, the ONS uses different age groups due to small sample sizes and estimates 78.0% of people aged 70 and over were likely to have tested positive for antibodies in the week to March 28.
An ONS spokesperson commented: “Our data shows that across all four UK nations at least one in two people would test positive for Covid-19 antibodies.
“By mid-March, we had begun to see a decrease in detectable antibodies amongst age-groups prioritised for vaccination. This trend appears to have reversed slightly, likely reflecting many of these same age groups now receiving their second dose of vaccine.
“There is a clear pattern between vaccination and testing positive for antibodies, however the detection of antibodies alone is not a precise measure of protection granted by vaccines.
"It is possible that antibody levels in some people are now too low to be detected by our tests but still high enough to grant a level of protection.
“We will continue to closely monitor the impact of vaccination in the coming months as restrictions ease.”
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