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.

“The number will only increase — 91 is just the beginning now,” said Kim Woo-joo, professor of infectious diseases at Korea University Guro Hospital, adding that patients had likely “relapsed” rather than been reinfected.

False test results also could be the cause, other experts said, or remnants of the virus could still be in people’s systems but not be infectious or of danger to the host or anyone else.

“There are different interpretations and many variables,” said Jung Ki-suck, professor of pulmonary medicine at Hallym University Sacred Heart Hospital.

“The government needs to come up with responses for each of these variables,” Jung said.

Almost 7,000 South Koreans have been reported as recovered from the illness, the news outlet reported.

On Friday, South Korea reported 27 new cases, its lowest after daily cases peaked at more than 900 in late February, according to KCDC.

The total stood at 10,450 cases and the death toll rose by seven to 211, it added.