"We observe a significant decline in the proportion of the population with detectable antibodies over three rounds of national surveillance, using a self-administered lateral flow test, 12, 18 and 24 weeks after the first peak of infections in England", the team wrote, according to CNN.
Scaled up to a nationwide level, it meant the proportion of the English population with antibodies dropped from 6.0 per cent to 4.4 per cent, according to the study.
"Regardless of the result of an antibody test, everyone must continue to comply with government guidelines including social distancing, self-isolating and getting a test if you have symptoms and always remember Hands, Face, Space".
The research comes as Britain announced 102 new deaths of people with Covid-19, bringing its official total to nearly 45,000, the highest toll in Europe.
"This is a really big challenge to the idea that herd immunity can be achieved through natural immunity", argued Helen Ward, professor of public health at Imperial College, and study co-author in comments to the Financial Times.
"We don't yet know what level of antibody is needed in a person's blood to protect them from infection or reinfection from SARS-CoV-2, but of course that level is a crucial thing to begin to understand".
Of course, even testing positive for antibodies is no guarantee that one is immune.
"Our study shows that over time there is a reduction in the proportion of people testing positive for antibodies", said professor Paul Elliott, director of the programme at Imperial College London.
It remains unclear what level of immunity antibodies provide, or for how long this immunity lasts, scientists said.
The study is yet another confirmation on studies which suggest that protection after COVID-19 infection may not be long-lasting.
"We thought that by mimicking the body's immune complement and coagulation proteins, coronaviruses may drive these systems into a hyperactive state and cause the pathology we see in infected patients", Shapira said. We don't yet know how long it lasts.
The study suggests that the immune system's response to the virus is similar to its reaction to influenza and other coronaviruses such as the common cold, which can be contracted seasonally.
Riley also notes this research should not be used to imply vaccine-induced immunity would be short-lived. "If you're not sleeping, no other lifestyle measure will make much difference", she says. "This study provides evidence that the level of immune response has declined over a relatively short period (three months) indicates that such future planning can not take for granted the beneficial effects of previous infection - importantly, should the results of this study prove robust, this implies that any strategy that relies on "herd immunity" lacks credibility".