More than 95% of England's population is unlikely to have the antibodies needed to protect them from coronavirus, new data suggests.
Research by Imperial College London estimated just 4.4% of adults had some form of immunity against Covid-19 as cases started to increase again in September.
This is compared with 6% found to have antibodies between June 20 and July 13, and 4.8% between July 31 and August 31.
According to the Real-Time Assessment of Community Transmission (REACT-2) study, there is a "variable waning in antibody positivity", meaning a possible decrease in immunity over time and an increased risk of reinfection as the number of antibodies decline.
Experts said the results show a vaccine is needed to protect large numbers of the population, and so-called "herd immunity" is still a "long, long way" off.
Some 365,104 adults took part in three rounds of testing for the study between late June and September to measure the prevalence of antibody positivity in England.
It found that those aged 18 to 24 had the highest prevalence of antibodies over the study period, which declined at the lowest rate out of all age groups.
People aged 75 and over had the lowest prevalence and the largest decline.
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."
The study found there was no change in antibody positivity in healthcare workers between June and September.
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."
In the latest round of the study, between September 15 and 28, the prevalence of antibodies remained highest in London, and in those of black and Asian ethnicity.
Jonathan Ball, professor of molecular virology at the University of Nottingham, said: "This study confirms suspicions that antibody responses – especially in vulnerable elderly populations – decrease over time.
"What is less clear from this kind of study is the relationship between waning immunity and susceptibility to reinfection and the resulting severity of any subsequent infection, and this is important to know."