Researchers at Public Health England (PHE) showed that reinfections in people who have COVID-19 antibodies from a past infection are rare. They found only 44 cases among 6,614 previously infected individuals.
However, experts warned that the findings meant that those who contracted the disease in the first wave of the pandemic - in the early months of 2020 - could now be vulnerable to catching it again.
They also warned that people with so-called “natural immunity” through having had the disease may still be able carry it in their nose and throat and could unwittingly pass it on.
“We now know that most of those who have had the virus, and developed antibodies, are protected from reinfection, but this is not total, and we do not yet know how long protection lasts,” said Susan Hopkins, co-leader of the research, known as the SIREN study.
“This means even if you believe you already had the disease and are protected, you can be reassured it is highly unlikely you will develop severe infections. But there is still a risk you could acquire an infection and transmit to others.”
The study involved tens of thousands of healthcare workers in Britain who have been tested regularly since June.