• The unnamed individual had other underlying medical conditions and sadly died in hospital in 2021.
• Most people naturally clear the virus, but the patient in question had a severely weakened immune system.
UK doctors believe they have documented the longest Covid infection on record - a patient they treated who had detectable levels of the virus for more than 16 months, or 505 days, in total.
The unnamed individual had other underlying medical conditions and sadly died in hospital in 2021.
Persistent infections such as this are still rare, say the London medics.
Most people naturally clear the virus, but the patient in question had a severely weakened immune system.
Chronic infections like these need studying to improve our understanding of Covid and the risks it can pose, say experts.
The patient first caught Covid in early 2020. They had symptoms and the infection was confirmed with a PCR test.
They were in and out of hospital many times over the next 72 weeks, for both routine checks and care.
On each occasion - about 50 in all - they tested positive, meaning they still had Covid.
The doctors, from King's College London and Guy's and St Thomas' NHS Foundation Trust, say detailed lab analysis revealed it was the same, persistent infection, rather than repeated bouts.
The patient could not shake the infection, even after being given antiviral drugs.
This is different to "long Covid", where the virus is cleared from the body but symptoms persist.
One of the medics who will be presenting the findings at a medical conference - the European Congress of Clinical Microbiology and Infectious Diseases - is Dr Luke Blagdon Snell.
He told the BBC: "These were throat swab tests that were positive each time. The patient never had a negative test. And we can tell it was one continuous infection because the genetic signature of it - the information we got from sequencing the viral genome - was unique and constant in that patient."
Prolonged infections are rare but important, say the researchers, because they might give rise to new variants of Covid - although that did not happen in this case.
Dr Snell said: "The virus is still adapting to the human host when people are infected for a long time. It might provide an opportunity for Covid to accrue new mutations.
"Some of these patients that we have studied have mutations that have been seen in some of the variants of concern."
He stressed that none of the nine patients they checked had spawned a new dangerous variant.
Someone with a chronic infection might not be contagious to others, he added.