The 42-year-old woman was fully inoculated with the BioNTech vaccine.
She had traveled to the US on August 24 and returned to Hong Kong on September 5.
The woman had given negative test results before departing for the US but tested positive upon arrival in Hong Kong.
Officials have classified the case as an imported one.
But as a precaution, health authorities have issued compulsory testing orders on her residential block, her workplace in Hung Hom as well as places she visited during her incubation period.