Coronavirus antibodies last at least six months and offer protection against a second infection, a study of healthcare workers suggests.
Staff at Oxford University Hospitals were regularly tested both for Covid-19 infections and for antibodies revealing a past infection.
The more antibodies people had, the lower their chances of re-infection.
A separate study found pre-existing immunity from other coronaviruses also protected against Covid.
Infection consultant Dr Katie Jeffery described the Oxford findings as “encouraging news” ahead of forthcoming Covid vaccines.