Immunity to coronavirus may only last a matter of a months, according to new research that could hinder the rollout of a successful vaccine.
A study by Imperial College London, which involved 365,000 people, showed that antibodies in the population fell by more than a quarter in just three months.
Scientists said the findings suggested a "rapid" decline in immunity – which could mean that even if a successful vaccine is found, it might have to be administered twice a year.
The mass research indicated that, by last month, fewer than one in 20 people had developed antibodies to Covid. Commissioned by the Department of Health, it is part of the largest piece of a research programme informing Government policies.
Its findings showed that by June, after the first wave of the pandemic, just six per cent of the population had developed antibodies, which suggest some level of protection against the virus. Three months later, that figure had dropped to 4.4 per cent, with most of the decline happening within just six weeks.
The sharpest fall was seen in those most in need of protection, with antibody levels among the over-75s reducing by close to 40 per cent between June and September.
Scientists said the findings showed Britain is "miles off" achieving herd immunity, which they warned might never be reached without a vaccine.
However, the research did not examine the role played by other forms of immunity. Some scientists believe the part played by T-cells – a type of white blood cell that helps the immune system fight off viruses and is linked with prior infections by common colds – could be more crucial in fighting the virus.
Scientists analysed home fingerprick test samples from hundreds of thousands of adults to establish "detectable antibody levels" over a period of three months, and found levels fell by 26.5 per cent overall.
The largest fall was among those most vulnerable to serious illness from Covid. Among those aged 75 and over, antibody levels fell by 39 per cent, while a drop of only 15 per cent was seen in those aged between 18 and 24.
Researchers stressed that it is not yet known what level of antibody response is required to protect against reinfection – meaning it is possible that even levels of antibodies that were not found by the tests were offering some protection. But they said the findings suggested a significant rapid decline in immunity, raising the prospect that those infected by Covid could suffer repeat infections in further waves.
The possibility that a vaccine may have to be administered as often as every six months increases the scale of the challenge ahead. However, the researchers said vaccines may prove more powerful than natural immunity.
On Monday, Matt Hancock, the Health Secretary, said the NHS and the Army were working on plans to roll out the vaccine programme as soon as a breakthrough is achieved.
He said he expected "the bulk of the rollout" to occur in the first half of next year, although he had not ruled out the possibility that some jabs might be administered before Christmas. NHS workers are expected to be among the first to be offered vaccines.
On Monday, the researchers warned that Britain is "a long, long way" from anything approaching herd immunity.
Helen Ward, a professor of public health at Imperial and the lead researcher, said: "I think what we are showing is that there is a really big challenge to that, which is that immunity is waning quite rapidly. After three months, we've already shown a 26 per cent decline in antibodies.
"When you think that 95 out of 100 people are unlikely to be immune, and therefore likely to be susceptible, then we are a long, long way, from anything resembling a population level protection against transmission."
Professor Wendy Barclay, the head of the Department of Infectious Disease at Imperial, said the findings suggested Covid is likely to work in the same way as the common cold, meaning it can reinfect regularly.
She said: "Seasonal coronaviruses that circulate every winter and cause common colds can reinfect people after six to 12 months – and we suspect that the way that the body reacts to infection with this new coronavirus is rather similar to that.
"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. Most of the vaccine strategies are aiming to produce that level, and that level will feed into whether or not a population becomes immune or has any level of immunity."
Prof Barclay warned that Britain was "miles off" herd immunity.
Graham Cooke, professor of infectious diseases at Imperial, said: "The big picture here is that, after the first wave, 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.
"People are starting to think about what we will need to do to boost vaccines periodically to keep levels of protection high, depending on what happens with those vaccines and what sort of strength of immune responses will be developed and how long they'll last for."
The health minister Minister Lord Bethell said the study was "a critical piece of research, helping us to understand the nature of Covid-19 antibodies over time and improve our understanding about the virus itself".
He added: "We rely on this kind of important research to inform our continued response to the disease, so we can continue to take the right action at the right time. It is also important that everyone knows what this means for them – this study will help in our fight against the virus, but testing positive for antibodies does not mean you are immune to Covid-19.
"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 remembering Hands, Face, Space."