The longest known Covid infection was identified in a Brit who had the virus for 505 days - longer than a year and four months - before dying.
Prior to this, the longest PCR confirmed case is thought to be 335 days long, or around 11 months.
Researchers also presented one of the first 'occult' Covid infections and evidence that new variants may arise in immunocompromised people.
Occult infections mean cases in which a person is thought to have cleared the virus through testing negative, but is later found to have an ongoing infection that hadn't been detected.
The findings stem from a study of nine patients in London with weakened immune systems due to organ transplantation, HIV, cancer, or medical therapies for other illnesses.
The research was conducted by a team from Kings College London and Guy's and St Thomas' NHS Foundation Trust.
Researchers were interested in how SARS-CoV-2 - the virus that causes Covid - changes over time in immunocompromised people.
As new variants have emerged in the pandemic, the team wanted to investigate which mutations arise, and if variants evolve, in these people with persistent infection.
"One theory is that these viral variants evolve in individuals whose immune systems are weakened from illness or medical treatments like chemotherapy, who can have persistent infection with SARS-CoV-2," said the study's first author, Dr Luke Blagdon Snell of Guy's and St Thomas' NHS Foundation Trust.
The participants tested positive for the virus for at least eight weeks with infections that had persisted for 73 days on average albeit two patients had persistent infections for more than a year.
Five of the patients, who were studied between March 2020 and December 2021, survived.
Whilst two of those five cleared Covid without treatment, two had treatment with antibody therapies and antivirals before overcoming the virus, and one has had an ongoing infection for 412 days.
One individual had Covid for 505 days before they died.
Regular sampling and genetic analysis of the virus showed that five of the nine patients developed at least one mutation seen in variants of concern.
Some individuals developed multiple mutations associated with variants of concern, such as the Alpha, Delta and Omicron variants.
The virus from one individual contained 10 mutations that would arise separately in variants of concern, such as the Alpha, Gamma and Omicron variants.
Dr Snell and colleagues said: "This provides evidence that mutations found in variants of concern do arise in immunocompromised patients and so supports the idea that new variants of the viruses may develop in immunocompromised individuals.
"It is important to note, however, that none of the individuals in our work developed new variants that became widespread variants of concern.
"Additionally, whilst this work shows variants could arise in immunocompromised individuals, whether the previous variants of concern like Alpha, Delta and Omicron arose in this manner remains unknown."
This research was presented at the European Congress of Clinical Microbiology & Infectious Diseases (ECCMID) in Lisbon, Portugal.
Don't miss the latest news from around Scotland and beyond - sign up to our daily newsletter here .
