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 around a 83% 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.