Singapore Prime Minister Lee Hsien Loong has announced that he tested positive for COVID-19 for a second time in less than two weeks, in a rare case of a rebound.

Lee, 71, was diagnosed with COVID-19 for the first time on May 22, following work trips to Africa and Asia. He tested negative six days later. But in a Facebook post on Thursday, Lee shared a photo of a positive antigen rapid test that he took.

"I feel fine but I am afraid I have turned COVID-19 positive again. My doctors say it is a COVID rebound, which happens in 5-10 percent of cases,” he said in his post.

Lee said he has been advised to self-isolate as the virus is still infectious, although the risk is lower compared to the initial infection. He said he had looked forward to attending the consecration ceremony of an Indian temple early on Thursday but has to "miss it to keep others around me safe.”

Lee will also have to skip a bilateral meeting on Friday with Australian Prime Minister Anthony Albanese, who is due to give a keynote address at the Shangri-La Dialogue security forum in Singapore. Albanese tweeted on Thursday that he called Lee to wish him a speedy recovery. Albanese said he looks forward to meeting instead with Lee's deputy Lawrence Wong.

Wong is tipped as successor to Lee, who has been in power since 2004.

In his earlier diagnosis, Lee said he was prescribed the Paxlovid antiviral medication due to his age. He didn't say if he was prescribed any medication this time. He has said his last COVID-19 vaccine booster was in November and urged Singaporeans to keep their vaccinations up to date to reduce the risk of severe illness.

Lee was on an official visit to South Africa from May 14 to 16, and Kenya from May 17 to 19. He also attended the Association of Southeast Asian Nations summit in Indonesia from May 10 to 11.

Following his first COVID diagnosis, Lee said his last vaccine booster was in November. He urged Singaporeans to keep their vaccinations up to date to reduce the risk of severe illness.

Health Minister Ong Ye Kung was reported by local media as saying recently in Parliament that fewer seniors were keeping up with their COVID-19 shots. He warned this could weaken the population’s resistance against COVID-19 over time and make the nation vulnerable to the virus again.