Levels of protective antibodies in people wane "quite rapidly" after coronavirus infection, say researchers.
Antibodies are a key part of our immune defences and stop the virus from getting inside the body's cells.
The Imperial College London team found the number of people testing positive for antibodies has fallen by 26% between June and September.
They say immunity appears to be fading and there is a risk of catching the virus multiple times.
The news comes as figures from the Office for National Statistics show that the number of Covid-19 deaths in the UK rose by 60% in the week of 16 October.
The ONS figures suggest there have now been more than 60,000 deaths involving Covid-19 in the UK.
More than 350,000 people in England have taken an antibody test as part of the REACT-2 study so far.