Most people who get coronavirus are safe from reinfection for at least six months, latest research suggests.
The finding follows the first large scale study of how much protection is offered by previously having had COVID-19.
It comes as optimism grows over the development and deployment of effective vaccines to combat the pandemic.
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However, while hailed as an important step to understanding coronavirus immunity, scientists say there is not yet enough data to make a judgement on how long it lasts beyond the six months from the initial infection and the work will continue.
The study, undertaken as part of a collaboration between the University of Oxford and Oxford University Hospitals (OUH) NHS Foundation Trust, covered a 30-week period (April - November) involving 12,180 health care workers.
The staff were tested for antibodies to the virus as a way of detecting who had been infected before.
During the study, 89 of 11,052 staff without antibodies developed COVID-19 with symptoms.
None of the 1,246 staff with antibodies developed a symptomatic infection.
Staff with antibodies were also less likely to test positive for COVID-19 without symptoms.
Some 76 staff without antibodies tested positive compared to just three who had them.
The three healthcare workers with antibodies who tested positive for the virus were all well and did not develop symptoms of COVID-19 again.
Scientists said the results indicated most people are unlikely to get COVID-19 again if they have already had it in the previous six months.
At the same time, health care workers who did not have antibodies against COVID-19 were more likely to develop the infection.
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One of the report's authors, Professor David Eyre of the University of Oxford's Nuffield Department of Population Health, said: "This ongoing study involving a large cohort of healthcare workers has shown that being infected with COVID-19 does offer protection against re-infection for most people for at least six months - we found no new symptomatic infections in any of the participants who had tested positive for antibodies, while 89 of those who had tested negative did contract the virus.
"This is really good news, because we can be confident that, at least in the short term, most people who get COVID-19 won't get it again.
"We know from a previous study that antibody levels fall over time, but this latest study shows that there is some immunity in those who have been infected. We will continue to follow this cohort of staff carefully to see how long protection lasts and whether previous infection affects the severity of infection if people do get infected again."
Dr Katie Jeffery, director of Infection Prevention and Control for Oxford University Hospitals said: "This is an exciting finding, indicating that infection with the virus provides at least short-term protection from re-infection - this news comes in the same month as other encouraging news about COVID vaccines."
