Coming into close contact with a Covid-19 sufferer yields a 20 per cent chance of catching the disease, a new study shows.
The large-scale survey, by Imperial College London, found antibodies in one in five people who said they had interacted with a positive case.
Overall, some 3.4 million people are now estimated to have been infected with coronavirus – about six per cent of the population.
The results come from the world's largest home testing programme looking for past evidence of the disease, using fingerprick testing kits.
The study tracked the spread of infection across England after the pandemic's first peak, with volunteers testing themselves at home between June 20 and July 13.
People living in London were most likely to have been infected, along with those working in care homes and health care, people from black, Asian and minority ethnic groups and people living in larger households.
The programme suggested a total of 13 per cent of people living in London had Covid-19 antibodies, compared with less than three per cent in the south-west of England.
People working in care homes (16 per cent) and healthcare (12 per cent) returned far higher results than people who were not key workers, at five per cent.
The study found that 17 per cent of black volunteers had antibodies, while the categories of Asian and other ethnic minorities had 12 per cent each. The figure among white volunteers was only five per cent.
People aged 18-34 showed the highest incidence of antibodies, at eight per cent, while the over-65s had the lowest rate, at just three per cent (the graphic below shows infections by age range during July and August).
Professor Graham Cooke, who led the research, said: "Clearly, the numbers are still very big and that explains the high levels of mortality we've seen. But as a proportion of the overall population it's still relatively small, except in some groups where the prevalence is higher, such as for people of Asian and black ethnicity.
"But obviously it means that if we had relaxation of the lockdown completely, we wouldn't have a very protected population so we would expect the virus to return, and so I think that has implications for decisions that are made."
The prevalence of antibodies in the population is far below the estimated 60 per cent minimum needed for herd immunity. However, the Government-backed survey cannot discern the prevalence of Covid-19 T-cells, which might offer another form of immunity.
Experts have predicted that T-cell prevalence – which can arise in reaction to other viruses, such as the common cold – would only need to be at 20 per cent in the population to achieve herd immunity.