A total of 91 patients who recovered from novel coronavirus in South Korea have tested positive again, health authorities said Friday, raising fears that the virus can be reactivated.

The Korea Centers for Disease Control and Prevention Director General Jeong Eun-kyeong said health authorities are conducting epidemiological and clinical studies into the cases to find out whether those who tested positive for the virus could infect others again.

Relevant testing of cultivating and isolating the virus could take about two weeks, she said.

The authorities have said that the virus might have remained dormant at undetectable levels in human cells and became reactivated, rather than patients becoming reinfected from someone else. There is also a possibility of wrong test results.

“A study into how immunity is developed after COVID-19 infection is still ongoing,” Jeong said at a briefing Friday.

Korea has seen the first case where a patient has tested positive after being declared cured on Feb. 9.

By Ock Hyun-ju (laeticia.ock@heraldcorp.com)