HONG KONG, Aug. 25 (Xinhua) -- Hong Kong reported Tuesday that a 33-year-old man had been confirmed to be infected with COVID-19 for a second time after his first infection in late March.

The man had been discharged from hospital after being cleared of the virus, but tested positive again after an overseas trip in August.

Researchers at the University of Hong Kong (HKU) said Tuesday that it is the first COVID-19 reinfected case in the world, suggesting that recovered COVID-19 patients are not immunized for life.

Researchers told Xinhua Tuesday that genetic sequencing showed the virus strains the man contracted in March and in August were distinctly different, which means the second infection was not caused by the residual virus from the first infection.

The researchers include Yuen Kwok-yung, chair professor of the Department of Microbiology of HKU, and To Kai-wang, clinical associate professor with the Department of Microbiology of HKU.

Yuen said vaccine is still the most pragmatic and important way to fight the pandemic, calling on the public to continue wearing masks, keeping social distancing and observing hand hygiene.