About 51 patients classed as having been cured in South Korea have tested positive again, the CDC said in a briefing on Monday. Rather than being infected again, the virus may have been reactivated in these people, given they tested positive again shortly after being released from quarantine, said Jeong Eun-kyeong, director-general of the Korean CDC.

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

A patient is deemed fully recovered when two tests conducted with a 24-hour interval show negative results.

The Korean CDC will conduct an epidemiological probe into the cases, Jeong said.