The research, commissioned by the British government, indicates people's immune response to COVID-19 reduces over time following infection. The findings also noted that samples from hundreds of thousands of people across England between mid-June and late September revealed that the prevalence of antibodies in people fell by more than a quarter.
Some scientists believe the part played by T cells - a type of white blood cell that helps the immune system fight viruses and which is linked with prior infections with common colds - may be more crucial in fighting the virus.
The study involved 365,000 randomly-selected adults administering at home three rounds of finger prick tests for coronavirus antibodies between June 20 and September 28.
Those aged 18-24 had the highest prevalence of antibodies and lowest decline in antibody levels at 14.9 percent.
Researchers stressed that it was not yet known what level of antibody response was required to protect against reinfection, meaning it was possible that even low levels of antibodies offered some protection. That raises the prospect of decreasing population immunity ahead of a second wave of infections that has forced local lockdowns and restrictions in recent weeks.
Researchers yesterday said the findings showed Britain was "a long long way" from anything approaching natural herd immunity.
Asked about herd immunity, Helen Ward, professor of public health at Imperial College London, said: "Even at best, (in the first round of the study) 94 per cent of the population remained not likely protected, and now that has declined to over 95 per cent of the population who don't have evidence of antibodies".
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The study found that antibody levels fell by 26.5 percent overall during the three-month period. The study is yet another confirmation on studies which suggest that protection after COVID-19 infection may not be long-lasting.
"We don't yet know whether this will leave these people at risk of reinfection with the virus that causes COVID-19, but it is essential that everyone continues to follow guidance to reduce the risk to themselves and others".
"So I think we are a long, long way from any idea that the population will be protected by other people".
She added: "These data should not be taken to infer that a vaccine would only induce short-term immunity".
Graham Cooke, professor of infectious diseases at Imperial College London, said: "The big picture here is that after the first wave (of coronavirus), the great majority of the country still did not have evidence of protective immunity".
The decline in people testing positive for antibodies was largest in those who did not report having had Covid-19, compared to those who said they previously tested positive for the virus.
"So the need for a vaccine is still very large if you want to try and get a large level of protection in the population".
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