At least 5% of people in the UK have now developed COVID-19 antibodies, with the number rising to 17% in London, a study has found.
Health Secretary Matt Hancock gave the results of the study at the government's daily coronavirus update, as he announced plans for antibody certificates.
The figures are the first ones to be released by a government-commissioned study run by the Office for National Statistics, using 1,000 adults to track levels of immunity in the UK.
Participants had to give blood samples that are tested to check how many had developed COVID-19 antibodies.
Experts are still unsure what level of immunity recovering from the disease gives people and how long it might last.
This, he said, is because someone with antibodies can be "safe and confident in the knowledge that you are most unlikely to get it again".
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The test, made by Swiss pharmaceutical firm Roche, has been approved for use by Public Health England.
Mr Hancock confirmed it will be rolled out for free on the NHS from next week – going to health and social care workers first.
However, experts are still unsure what level of immunity recovering from the disease gives people – and how long the immunity might last.
John Edmunds, professor of infectious disease modelling at the London School of Hygiene and Tropical Medicine, has warned studies of other coronaviruses suggest "potentially bad news" for hopes humans could develop long-term immunity.
He told a parliamentary committee on Tuesday: "We can also see from other coronaviruses, from ones that cause coughs and colds, that individuals again do seem to not have particularly long-term immunity to many of those viruses, allowing them to get reinfected later.
"Immunity may not last that long against this virus."
There have been more than 252,000 confirmed cases of COVID-19 in the UK, and at least 36,124 deaths.