TORONTO -- A woman in the U.S. never developed any symptoms of COVID-19 even though the novel coronavirus was in her system for at least 15 weeks, according to a new case study.

The 71-year-old woman was hospitalized in late February due to severe anemia. When doctors realized that she had just spent a week post-surgery at a rehab facility in Kirkland, Wash. that was experiencing a large COVID-19 outbreak, they tested her for the virus.

She tested positive for SARS-CoV-2 on March 2. And then she kept testing positive. As days turned into weeks, well past the time when most patients have shed the virus, test results showed that it was still in the woman's system.

In total, the woman tested positive 14 times through June 15. She then tested negative four times over the next month, indicating that the virus had finally left her system.

By processing samples from the woman's upper respiratory tract, researchers found that the woman had the virus in her system for at least 105 days before it cleared, and was shedding it – the period during which a person is contagious – for at least 70 days. A more typical shedding period for SARS-CoV-2 is eight days, and according to the case study, the previous longest-known shedding period was 20 days.

The leading theory is that because the woman is severely immunocompromised – she has leukemia and a low antibody count – her immune system was never able to fight back against the invading threat, not even when doctors attempted to kickstart a response with a convalescent plasma treatment. Blood tests showed that the woman's body never produced any antibodies to fight the virus.

"Although it is difficult to extrapolate from a single patient, our data suggest that long-term shedding of infectious virus may be a concern in certain immunocompromised patients," doctors who monitored the woman wrote in the case study, which was published Wednesday .

It is not known what happened to cause the virus to finally leave the woman's body, 105 days after it was first detected.

Curiously, though, the woman never developed any symptoms of COVID-19. In fact, she's believed to be the longest known case of asymptomatic COVID-19.

This makes her unique among the "long-haulers" of the pandemic, most of whom have reported persistent and sometimes life-altering symptoms, but fits with past documented cases of influenza and the MERS respiratory illness.

"This is something that we expected might happen, but it had never been reported before," case study author Vincent Munster .