South Korea authorities have announced that at least 51 patients classed as “cured” have tested positive for the virus again, raising concerns about how immunity to the diseases is developed, as well as the potential for a vaccine.

So far, doctors are not sure why this happens, but they believe it is a case of reactivation inside the body rather than reinfection.

“While we are putting more weight on reactivation as the possible cause, we are conducting a comprehensive study on this,” said Jeong Eun-kyeong, director-general of the Korean CDC. “There have been many cases when a patient during treatment will test negative one day and positive another.”

South Korea was one of the earliest countries to see an outbreak, with a religious gathering working as the triggering event. Since then, however, South Korea has gained substantial praise for its handling of the outbreak, especially through its capacity for mass-testing and producing masks and other protective equipment. South Korea is also at the vanguard of COVID-19 research, which is why these recently-announced results have raised significant concerns.

It’s not the first time fears of reinfection emerged. Reports coming from China all the way in February raised the problem of reinfection, but it never became clear if this is truly caused by reinfection, reactivation, or imperfect tests.

Like many other countries, South Korea uses a reverse transcription-polymerase chain reaction (RT-PCR) test to find genetic traces of the virus in a sample taken from a patient. Remarkably, the KCDC found that traces of the virus still remained in 25 patients, even after antibodies had formed.

“It has turned out all the 25 had neutralising antibodies against the Covid-19 virus,” the KCDC director said on Wednesday, referring to the central part of the adaptive immune system that defends a cell from pathogens.

“However, 12 people of the 25 people tested positive for the virus’ RNA in [pharyngeal-nasal swab] tests. We assume that it takes different times for the virus to be removed from different patients after antibodies occur.”

Fear of reinfection or reactivation in recovered patients is also growing in China, where the virus first emerged last December, after reports that some tested positive again — and even died from the disease — after supposedly recovering and leaving the hospital. There’s little understanding of why this happens, although some believe that the problem may lie in inconsistencies in test results.

The Korean CDC (KCDC) is carrying a larger-scale study to look into this but, for now, reinfection seems to be quite unlikely, which leaves either testing or reactivation as the likely culprits. Whichever the case may be, it’s a significant problem.

If it’s the testing, it means that a lot of the negative test results might have not been negative after all (though this is still probably a minority of tests). If it’s an issue regarding how antibodies deal with the virus, it could have long-term implications for not only the disease itself, but for how a vaccine can be developed, and how it can be effective long-term. Eom Jung-sik at Gacheon Hospital said this would undermine efforts to develop an antibody-based vaccine and other treatment.

“It is not the case that all antibodies can control virus and should COVID-19 virus remain alive even after antibodies are formed, it could surge back, causing a relapse in symptoms,” he said. “It would take few years to clarify how efficient neutralising antibodies are in combating the virus.”

For now, epidemiologists all around the world are racing to find out more about COVID-19. It seems like it’s been forever, but we’re still in the early stages of what seems to be a marathon race — and in this race, information is one of our best allies.