Sixteen more people who had recovered from coronavirus and were released from quarantine have tested positive again for the infection, South Korea’s Centers for Disease Control and Prevention (KCDC) reported on Sunday.

This means that 179 people in total have retested positive after they were released from quarantine, out of 8,042 patients who’ve recovered from Covid-19 so far, KCDC director Jung Eun-Kyeong told a press briefing.

It is currently unclear why patients could be retesting positive. Most experts think it’s unlikely that somebody will be reinfected right after recovering. It’s possible that issues with testing — or varying amounts of viral RNA in the body, which the tests look for — could explain why people test positive after testing negative, experts say.

An in-depth epidemiological investigation is underway to figure out the cause, Jung said.

Among all of the cases who have retested positive, patients in their 20s made up the highest number, with 41 cases (22.9%), followed by patients in their 50s, with 32 cases (17.9%), according to the KCDC.

Most such cases test positive after an average of about 13 days following release from quarantine, Jung added, and so far no secondary infections have been reported from these cases.