South Korea has reported over 100 cases where people have recovered from COVID-19 but since tested positive for the disease again.

While the observation has raised confusion and some anxiety, health authorities have said it’s most likely that the novel coronavirus was "reactivated" in the patients, rather than the people being reinfected. Alternatively, it could also point towards some kind of issue with testing and sampling.

Korea Centers for Disease Control and Prevention (KCDC) reported only 25 new confirmed cases of COVID-19 on Monday, April 13. However, speaking at a regular press conference that day, the director of the KCDC said a total of 116 patients had tested positive again after testing negative and being released from quarantine, according to South Korean state-owned Yonhap News Agency.

"Investigators are looking into whether the cause of relapse is the virus being reactivated or reinfection with the virus. We are also investigating whether relapse cases lead to secondary infection," Jeong Eun-kyeong, director of the KCDC, told reporters on April 13, per Yonhap.

Other countries have also reported cases of people testing positive for COVID-19 after appearing to recover from the disease. Back in February 2020, there were early cases of some people in China suspected of relapsing with the infection after recovery, as well as similar reports of a woman in Japan testing positive for a second time weeks after recovery.

The big questions are whether or not people are at risk of falling ill again and whether they transmit the infection to others. In short, no one knows yet. As with many aspects of this novel disease, the scientific community has little in the way of pre-existing knowledge on COVID-19 and the planet’s understanding is constantly evolving.

The US Centers for Disease Control and Prevention (CDC) is uncertain whether recovered COVID-19 patients can be re-infected, saying: “The immune response, including duration of immunity, to SARS-CoV-2 infection is not yet understood. Patients with MERS-CoV [a related coronavirus] are unlikely to be re-infected shortly after they recover, but it is not yet known whether similar immune protection will be observed for patients with COVID-19.”

With that said, some experts have suggested that people testing positive, then negative, before testing positive again could be a technical issue with the testing and sampling, rather than evidence of a “reinfection.”

“There have been a few isolated examples where [reinfection] has been reported. That people were positive, then they were negative, then they were positive again. It looks like, in the great majority of cases, this doesn’t happen. That people get infected once,” Professor Jimmy Whitworth, an expert in international public health from the London School of Hygiene and Tropical Medicine, said in a live Q&A on March 25.

“My suspicion is that those discrepant test results that we get are to do with, actually, the sampling. It’s not straightforward to take a sample from the back of the throat and make sure you catch virus every time,” explained Professor Whitworth.

“So I suspect it’s a technical issue, rather than repeat infection… It looks like you are immune for getting it again, but for how long, we don’t know yet.”