While most people infected with the novel coronavirus actively shed the pathogen for about eight days, scientists have reported an unusual case of a blood cancer patient who carried the virus for about 105 days, "and remained infectious for at least 70," without experiencing any symptoms the entire time.
According to the study, published in the journal Cell, understanding how long people can remain actively infected is important since it provides new details about COVID-19 that are still not well understood.
"At the time we started this study, we really didn't know much about the duration of virus shedding," said study senior author Vincent Munster, a virologist at the National Institute of Allergy and Infectious Diseases in the US.
"As this virus continues to spread, more people with a range of immunosuppressing disorders will become infected, and it's important to understand how SARS-CoV-2 behaves in these populations," Munster said.
The patient from Kirkland, Washington, had been infected very early in the COVID-19 pandemic and had had numerous positive PCR tests for the virus over a period of weeks, the study noted.
According to the researchers, the patient, a 71-year-old woman, was immunocompromised due to chronic blood cancer but never showed any symptoms of COVID-19.
She was found to be infected with the virus when she was screened after being admitted to the hospital for severe anemia, and her doctors recognised that she had been a resident of a rehabilitation facility experiencing a large outbreak.
As the researchers studied samples that were regularly collected from the patient's upper respiratory tract, they found that infectious virus continued to be present for at least 70 days after the first positive test, and the woman didn't fully clear the virus until after day 105.
"This was something that we expected might happen, but it had never been reported before," Munster said.
The researchers believe the patient remained infectious for so long because her compromised immune system never allowed her to mount a response.
They found from blood tests that her body was never able to make antibodies.
According to the scientists, even treatment with antibodies from recovered COVID-19 patients, had little effect on her health.
Despite her inability to mount an antibody response, she never went on to develop COVID-19, they added.
When the scientists examined the patient to see how the virus might have changed over the course of the infection, they found that samples collected from her at various times displayed different dominant gene variants of the virus.
However, the researchers believe these mutations did not play a role in how long the virus persisted since they saw no evidence of evolution of the pathogen.
According to Munster, this could be the longest case of anyone being actively infected with the coronavirus while remaining asymptomatic.
"We've seen similar cases with influenza and with Middle East respiratory syndrome, which is also caused by a coronavirus. We expect to see more reports like ours coming out in the future," he said.