Researchers at the University of Hong Kong have confirmed the world’s first case of COVID-19 reinfection. A 33-year-old IT worker from Hong Kong who recovered from the virus and was discharged from a hospital in April has again been tested positive after returning from Spain earlier this month.

The man’s second SARS-CoV-2 infection was detected at the airport screening on his return to Hong Kong.

Researchers say that they used “genomic sequence analysis to prove that he had been infected by two different strains”. The information technology worker didn’t develop any symptoms from his second infection, which might indicate any “subsequent infections may be milder,” reported Bloomberg.