The first person in the world known to have harboured the Covid virus for more than a year was identified by doctors in the UK.

A patient with a weakened immune system died after they tested positive for Covid for more than 500 days. A US cancer survivor in her 40s tested positive for Covid for 335 days.

The researchers at King's College London and Guy's and St Thomas' looked at the evolution of the virus in nine patients with persistent Covid. All of the patients had weakened immune systems due to organ transplantation, HIV, cancer or therapies for other illnesses.

Two patients had the virus for more than a year after the infections ended. A second patient has tested positive for more than one day and may exceed the 500 day record at their next appointment. Dave Smith, a 72 year old patient, had tested positive for over a year.

Patients with weakened immune systems are more vulnerable to Covid infections. Four of the nine patients died, with Covid contributing a third to half of those deaths. New Covid variants may be a result of persistent infections because the virus acquires new genes as it progresses in the patient.

The individuals seem to have a poor outcome when they have persistent infections. It may have an added benefit of preventing the emergence of variant.

In five of the nine patients, the virus had at least one variant of concern that is prone to cause major waves of disease. The patient who was the one who got the virus had 10 different variations of it, including Alpha and Omicron.

Some scientists think that Alpha may have arisen in patients with persistent infections, but other sources of new variants are possible, such as animals that contract the virus and then pass it back to humans. The study's patients appeared to have not spread beyond the people they were with.

One patient in the study is likely to have had a rare occult, meaning they tested negative for Covid despite having an ongoing infection. After falling ill with the Alpha variant in 2021, the patient's symptoms subsided and they tested negative several times, but later saw the Alpha variant bounce back and cause further symptoms, despite the variant no longer being present in the UK. The patient did not travel outside the country. The virus may have been hiding in their lungs, where it could not be detected with nose and throat samples.

New treatment strategies are needed to help patients clear persistent infections. The work will be presented at the European Congress of Clinical Microbiology and Infectious Diseases in Lisbon on Friday.

Paul Hunter, a professor of medicine at the University of East Anglia who was not involved in the study, said there may be some people who never clear their Covid infections. The effectiveness of the antibody treatments in clinics today is less than in the past.