The patient is a 56-year-old man who arrived from the United Kingdom on Monday.
He received a booster BioNTech jab last month, after being double-jabbed with AstraZeneca in February and April in the UK.
Meanwhile, the Centre for Health Protection said it turned out that an 18-year-old Hong Kong man who was said to have Covid-19 after travelling to Singapore last month was not infected after all.
Authorities in Singapore said further checks showed it was a false positive result.
The centre said his close contacts in Hong Kong who were sent to the quarantine camp earlier are therefore allowed to leave.