Most people who have had COVID-19 are protected from catching it again for at least five months, a study led by Public Health England shows.
Past infection was linked to an 83 percent lower risk of getting the virus, compared with those who had never had COVID-19, scientists found.
But experts warn some people do catch COVID-19 again - and can infect others.
And officials stress people should follow the stay-at-home rules - whether or not they have had the virus.
Prof Susan Hopkins, who led the study, said the results were encouraging, suggesting immunity lasted longer than some people feared, but protection was by no means absolute.
It was particularly concerning some of those reinfected had high levels of the virus - even without symptoms - and were at risk of passing it on to others, she said.
"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 that you could acquire an infection and transmit to others," she added.
"Now more than ever, it is vital we all stay at home to protect our health service and save lives."
From June to November 2020, almost 21,000 healthcare workers across the UK were regularly tested to see whether they:
- currently had the coronavirus
- had had it previously
Of those who had no antibodies to the virus, suggesting they had never had it, 318 developed potential new infections within this timeframe, the tests indicated.
But among the 6,614 with antibodies, this figure was just 44.
Scientists will continue to monitor the healthcare workers for 12 months to see how long immunity lasts.
They will also look closely at cases with the new variant - which was not widespread at the time of this first analysis - and observe the immunity of participants who receive the vaccine.
media captionCan you become immune to coronavirus?
Dr Julian Tang, a virus expert at the University of Leicester, said the results were reassuring for healthcare workers.
"Having the vaccine after recovering from Covid-19 is not an issue... and will likely boost the natural immunity," he added.
"We also see this with the seasonal flu vaccine.
"So hopefully the results from this paper will reduce the anxiety of many healthcare-worker colleagues who have concerns about getting COVID-19 twice."