The patient first tested positive in early 2020 after presenting symptoms of COVID-19. Then over the course of 16 months, the patient was in and out of the hospital—repeatedly taking PCR tests, which all came back positive.

A detailed lab analysis revealed the patient had the same, persistent COVID, rather than repeated bouts of the disease. And even after being administered antibody therapies and antiviral medication, the patient never once tested negative.

The patient, much like others who have persistent COVID, was immunocompromised with underlying medical conditions and later died at the hospital in 2021.

The doctors from King’s College London and Guy’s and St Thomas’ NHS Foundation Trust, presenting the case at this year’s European Congress of Clinical Microbiology & Infectious Diseases in Lisbon, note the previous longest known persistent COVID-19 case was 335 days.

What this means

Persistent COVID differs from the well-known long COVID, where people who have been infected with SARS-CoV-2 experience long-lasting symptoms like shortness of breath and fatigue for weeks or months after the initial infection is gone.

In persistent COVID, an infected person will continue to replicate the same disease within their bodies for far longer than the one to two weeks normally experienced after first exposure.

Luke Blagdon Snell, a doctor at the Department of Infection at Guy’s and St Thomas’ NHS Foundation Trust who is presenting the findings, told Fortune, “What we’re talking about when we talk about persistence is continued viral replication within the human body.” He adds, “The virus is continually replicating the entire time, so it really is an ongoing infection.”

While persistent COVID is very rare and likely to affect only a very small percentage of the population, it can lead to new mutations in the virus as infected patients incubate and spread new strains of SARS-CoV-2. Doctors theorize that new variants of COVID-19 can evolve in immunocompromised people who have persistent COVID infections.

In the study, which followed the patient who had COVID for 505 days, nine other patients who also had persistent COVID were examined.

What the doctors found after regular sampling and genetic analysis is that five of the patients developed at least one of the mutations seen in the variants of concern and one patient even developed 10 new mutations that arose separately from variants of concern, like Alpha, Gamma, and Omicron variants. The doctors note the findings suggest new mutations do develop in immunocompromised individuals with persistent, regularly replicating COVID.

Of the nine patients, who also had weakened immune systems due to organ transplantation, HIV, cancer, or medical therapies for other illnesses, only five survived. Two cleared the infection without treatment, two others cleared the infection with antibody therapies and antivirals, and one still has COVID after 412 days of infection.

Moving forward, Snell urgently calls for research into new treatment strategies for clearing persistent COVID.

“We don’t really know the best way for persistent infection to clear that virus. We need ways to stop that virus replicating.”