Most people who have had coronavirus are protected from catching it again for at least five months, according to a major UK study.
The Public Health England (PHE) study of healthcare workers found that an infection gave 83 per cent ongoing protection from reinfection.
Of 6,614 health workers who were previously infected, only 44 caught the virus again within five months, the findings showed.
Scientists said the findings were not a green light to lift restrictions because those who developed antibodies could still carry the virus and spread it to others.
However, a leading epidemiologist suggested isolation restrictions on healthcare workers could be eased, allowing them to return to work faster, taking pressure off the National Health Service.
Officials emphasised that it was essential people continued to follow social distancing rules even if they have had the virus.
And they cautioned that the findings showed that people who caught the disease in the first wave of the pandemic in the early months of 2020 may now be vulnerable to catching it again.
Prof Susan Hopkins, who led the study, said the results were encouraging, but was concerned that those who had been previously infected still had high levels of the virus in their nose and throat.
"It's absolutely not a green light," she told BBC's Radio 4 Today programme.
“It’s the same rules we're asking of people who’ve had the vaccine. It reduces the risk of infection and reduces the risk of severe disease, but some people still carry this and can transmit to others.”
Prof Neil Ferguson, an epidemiologist at Imperial College London, said the findings could be beneficial for healthcare workers who are forced to self-isolate.
“We have a real problem at the moment of healthcare workers getting infected and being off work,” he said.
“Whether we can relax restrictions temporarily on isolation [for those] who’ve had the virus in the past few months is a question for policymakers. But it could ease pressures, for instance, on the health service.”
A statement on the study said its findings did not address antibody or other immune responses to vaccines now being used against Covid-19, or on how effective vaccines would be. Vaccine responses will be considered later this year.
The research, known as the Siren study, involves tens of thousands of healthcare workers in the UK who have been tested regularly since June for new Covid-19 infections as well as for the presence of antibodies.
Between June 18 and November 24 scientists detected 44 potential reinfections – two probable and 42 possible – out of 6,614 participants who had tested positive for antibodies. This represents an 83 per cent rate of protection from reinfection, they said.
The researchers plan to continue following and assessing the participants to see if this natural immunity might last longer than five months in some. But they warned that early evidence from the next stage of the study already suggests some people with immunity can still carry high levels of virus and could transmit it to others.
"It is therefore crucial that everyone continues to follow the rules and stays at home, even if they have previously had Covid-19," they said.
Experts not directly involved in the research urged people to note its key findings.
"These data reinforce the message that, for the time being, everyone is a potential source of infection for others and should behave accordingly," said Eleanor Riley, a professor of immunology and infectious disease at Edinburgh University.
Simon Clarke, an associate professor in cellular microbiology at Reading University, said the study "has major implications for how we can get out of the current crisis".
"This means that the vast majority of the population will either need to have natural immunity or have been immunised for us to fully lift restrictions on our lives, unless we are prepared to see many more people being infected and dying from COVID-19," he said.
