Video report by ITV News Health Editor Emily Morgan
Covid-19 immunity could only last a few months following infection, after a study revealed a decline in protective antibodies.
Research by Imperial College London estimated just 4.4% of adults had some form of immunity against Covid-19 in September, when cases began to increase again.
This is compared with 6% found to have antibodies between June 20 and July 13, and 4.8% between July 31 and August 31.
Findings published by Imperial College London and Ipsos MORI suggest that people who did not show symptoms of the virus were likely to lose antibodies sooner than those who did show symptoms.
Those aged 18-24 had the highest prevalence of antibodies and lowest decline in antibody levels at 14.9%.
Meanwhile, people aged 75 and over had the lowest prevalence and saw the largest drop, with antibody levels falling by 39%.
Researchers warned, however, that it remains unclear whether antibodies provide any effective level of immunity or, if such immunity exists, for how long it might last.
Surveying more than 365,000 participants across England, researchers found the number of people with antibodies to coronavirus fell by 26.5% over three months.
Dr Sarah Jarvis on the significance of the antibody study
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.
“So although we are seeing a decline in the proportion of people who are testing positive, we still have a great majority of people who are unlikely to have been exposed.
“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.”
Candidates in the study tested themselves at home using a finger prick test between 20 June and 28 September to check if they had antibodies against coronavirus.
While the number of people testing positive for antibodies declined gradually in the population regardless of employment type - the number of health care workers testing positive for antibodies didn’t change over time.
Helen Ward, professor of Public Health at Imperial College London, said this could indicate “ongoing transmission” of coronavirus in those settings or “repeated exposure”.
Asked about herd immunity, Prof Ward said: “Even at best, (in the first round of the study) 94% of the population remained not likely protected, and now that has declined to over 95% of the population who don’t have evidence of antibodies.
“So I think we are a long, long way from any idea that the population will be protected by other people.”
She added that immunity in England was “waning quite rapidly”.
In the latest round of the study, between September 15-28, the prevalence of antibodies remained highest in London, and in those of black and Asian ethnicity.
The results of all three rounds of antibody testing indicate the first wave of the epidemic occurred over a relatively short period in March and April.
While it suggested a steep decline in the proportion of people who reported having Covid symptoms and who tested positive for antibodies from early April - two weeks after national lockdown.
Health Minister Lord Bethell stressed "testing positive for antibodies does not mean you are immune to Covid-19".