Commuters wearing a face mask or covering due to the COVID-19 pandemic, walk past a London underground tube train at Victoria station, during the evening ‘rus hour’ in central London on September 23, 2020.TOLGA AKMEN | AFP via Getty ImagesLONDON — Antibodies against the coronavirus fall as people recover from the disease, according to the findings of a major U.K. study, potentially dealing a blow to those pushing for so-called herd immunity.Researchers from Imperial College London screened 365,000 people in England over three rounds of testing between June 20 and September 28.Analysis of finger-prick tests carried out at home found that, rather than people building immunity over time, the number of people with antibodies that can fight Covid-19 declined roughly 26% over the study period.The REACT-2 study, which has not yet been peer reviewed, found that 6% of those tested had antibodies to the virus when the U.K.’s lockdown measures were relaxed over the summer. However, by the start of the second wave of cases last month, this figure had fallen to 4.4%.”This very large study has shown that the proportion of people with detectable antibodies is falling over time,” said Helen Ward, one of the authors of the study and professor at Imperial College London.”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.”What does it mean for herd immunity?The findings suggest that there may be a decline in the level of population immunity in the months following the first wave of the coronavirus epidemic, potentially dashing the hopes of those calling for a controversial herd immunity response strategy.Herd immunity occurs when enough of a population is immune to a disease, making it unlikely to spread and protecting the rest of the community, according to the Mayo Clinic. It can be achieved through natural infection — when enough people are exposed to the disease and develop antibodies against it — and through vaccinations.Health experts estimate that around 70% of the population would need to be vaccinated or have natural antibodies to achieve herd immunity.A man wearing a protective face mask, shelters from the rain under an umbrella as he walks past Chancery Lane underground station in London on October 21, 2020, as the government considers further lockdown measures to combat the rise in novel coronavirus COVID-19 cases.JUSTIN TALLIS | AFP via Getty ImagesSome epidemiologists have suggested that aiming for herd immunity would be a better response to the pandemic than lockdown measures. Many others, however, have sharply criticized a strategy that could require vulnerable people to shield at home while the virus spreads through the young and healthy.Earlier this month, Dr. Anthony Fauci, the U.S.’s top infectious disease expert, described calls to let the virus rip through the U.S. population unchecked as “nonsense” and “dangerous.”To date, more than 43.5 million people around the world have contracted the coronavirus, with 1.16 million related deaths, according to data compiled by Johns Hopkins University.Implications for reinfectionThe results of the REACT-2 study showed a downward trend of antibodies in people of all age groups and in all areas of the U.K., but not in health workers. The decline was largest for people aged 75 and above, the study said, while the smallest fall was among those aged between 18 to 24 years old.Researchers found that the decline in prevalent antibodies may initially be rapid, before plateauing. They cautioned that data on this was only now beginning to emerge.The study only measured antibodies. The authors said it was not possible to determine whether the loss of antibody positivity would correlate with an increased risk of an individual being reinfected since it was not clear what contribution T cell immunity and memory responses played in protective immunity during re-exposure.T cells are part of the immune system that defends against specific foreign pathogens.Late-night drinkers after 10pm in Soho, London, after Prime Minister Boris Johnson announced that from Thursday pubs and restaurants will be subject to a 10pm curfew to combat the rise in coronavirus cases in England.Yui Mok – PA Images | PA Images | Getty Images”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 Real Time Assessment of Community Transmission program at Imperial, and one of the authors of the study.”Testing positive for antibodies does not mean you are immune to COVID-19. It remains unclear what level of immunity antibodies provide, or for how long this immunity lasts,” he continued.”If someone tests positive for antibodies, they still need to follow national guidelines including social distancing measures, getting a swab test if they have symptoms and wearing face coverings where required.”— CNBC’s Noah Higgins-Dunn contributed to this report.