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The World Health Organization warned there's currently no evidence that people who've recovered from the coronavirus are protected from a second infection. More than 260 patients in South Korea have retested positive after it was thought they had recovered, according to the country's Centers for Disease Control and Prevention, raising questions about whether the virus can be reactivated.
Colorado nurse practitioner Lisa Merck tested positive for coronavirus in March. As a health care worker, Merck gets repeat tests, so she knows when she can safely return to work.
On day 21 of isolation, she tested negative. But, just seven days later, she got a retest and was shocked at the result.
"This is really scary to me," she said. Merck was once again positive for COVID-19.
"It's really confusing. It's very frustrating," Merck told CBS News senior medical correspondent Dr. Tara Narula.
Merck said she thinks the negative test was a false negative, but said, "there's no way to tell" if it was or if she got infected with coronavirus again.
Viral PCR tests based on nose and throat swabs like the ones for coronavirus are reliable about 60% to 90% of the time, depending on the test.
Asked if people can get COVID-19 twice, Dr. Stanley Perlman, who has been studying coronaviruses like SARS and MERS for nearly 40 years, said, "I don't think we know the answer to that yet."
Perlman, a professor at the University of Iowa, said about 50% of people who had a SARS infection still have antibodies now, 17 years later.
"So that would tell you, maybe most of them are protected against SARS again," he said.
For MERS, he said, "the amount of antibody seems to have declined fairly rapidly if you have mild disease. If you have severe disease, it lasts much longer."
Just like with MERS, Perlman believes more severe COVID-19 symptoms may mean longer-lasting immunity.
"We think that you will be protected at least for some amount of time," he said. "Especially if you had pneumonia. If you have only the upper airway infection, you may get infected again."
Perlman said the good news is that COVID-19 is not mutating quickly. "So, this means that making a vaccine is more feasible because the virus isn't changing," he said.
Responding to the data from South Korea about people retesting positive for the virus, Perlman said, "The question is whether it really is reactivation or it's low-level infection that was not detected for a period of time and now is detected."
Martin Hirsch, of Massachusetts General Hospital, agreed. Genetics and several other factors affect immune response, he said.
"Younger people, healthy people without any underlying conditions are the most likely to develop prolonged antibodies," he said.
With COVID-19, the WHO said it needs more evidence about how effective the antibodies are and how long immunity will last.
"I think those people who test positive, then negative and then positive within a few days or a few weeks are almost certainly not reinfected," Hirsch said. "I think if you go beyond that ... a few months, a few years, then you can start talking about reinfection."