Immunity to Covid-19 may only last for a matter of months, raising the prospect of a potential vaccine having to be administered more than once a year.
A large-scale study carried out by Imperial College London, involving 365,000 people, showed antibody levels dropped by a quarter in three months.
Researchers said the results could point to a decline in the immunity of the population, leading to a heightened risk of reinfection.
They stressed that those who have had a confirmed Covid-19 infection should take precautions to protect themselves from contracting it again.
Antibodies produced by the immune system are a crucial line of defence against viruses.
“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, which carried out 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.”
The findings build on previous research, which has also found evidence of waning antibodies.
One recent study from researchers at the University of Montreal showed antibody levels in the blood "drop rapidly" after infection.
The Imperial College study involved finger-prick blood tests to check for evidence of past infection in more than 365,000 people.
The tests are randomly and routinely sent to adult volunteers to be carried out at home.
There were 17,576 positive results recorded in three rounds of testing during the study.
Antibody prevalence dropped from 6 per cent, to 4.8 per cent and then 4.4 per cent during the three month period.
The decline in antibodies was seen in all areas of the country and across all age groups, except healthcare workers.
Researchers said that may suggest they received repeated or higher initial exposure to the virus, mounting a stronger immune response.
Studies have shown asymptomatic infections of Covid-19 tend to produce low, and in some cases, even undetectable antibodies, while those who suffer more symptoms mount a stronger immune response.
Higher antibodies appear to last longer.
Among age groups, the smallest drop was seen in the youngest age group, age 18 to 24, while the largest decline was seen in the oldest age group, 75 and above, who have weaker immune systems.
There have been a handful of reinfections of Covid-19 recorded so far.
It is not known how often they occur as experts have said they are likely to be an underestimate, as it requires virus lineages to be sequenced each time, which is not routinely done.
Of the four reinfections cases studied by researchers, symptoms were worse in two patients, suggesting no clear pattern.
Experts have said if SARS-Cov-2, which causes Covid-19, follows the same pattern as other coronaviruses, reinfections will become common.
Research has shown it is possible to catch a cold caused by the same strain months later as immunity fades quickly.
In one study from Kenya in 2018, almost 30 per cent of those who caught one variant of a coronavirus experienced a second reinfection. Around 10 per cent caught it a third time and one person was infected four times.
A number of reinfections occurred only three months after the first bout, and in multiple cases the viral load actually increased, “revealing ineffective protective immune responses after initial exposure”.