The number of people in South Korea who tested positive for the coronavirus for a second time after apparently recovering has risen to 91 — an increase of 40 from the 51 reported earlier this week, according to a report.

Jeong Eun-kyeong, head of the Korea Centers for Disease Control and Prevention, told reporters that the virus may have been “reactivated” rather than the patients being reinfected, Reuters reported.

Health authorities in the country said epidemiological investigations were under way to figure out what is behind the disturbing trend — as many countries are hoping that people will develop sufficient immunity to prevent a resurgence of the pandemic.