All of them carry the potentially more contagious L452R variant of the coronavirus.
The Centre for Health Protection said one of the patients, a 53-year-old man who returned from Namibia, had received two doses of BioNTech vaccine, in March and April, in Hong Kong.
Another patient, a 42-year-old woman from Cyprus, received two doses of the Sputnik V vaccine in March and April, in Moscow.
Microbiologist Ho Pak-leung of the University of Hong Kong said an increasing number of people who are fully vaccinated are catching Covid globally, and he expects more inbound travellers who are vaccinated to be infected.
To prevent coronavirus from leaking into the community, Ho said it is therefore all the more important to fully vaccinate everyone who works at the airport.
Ho added that despite the so-called “vaccine breakthrough cases”, the Covid-19 shots are still very effective in preventing complications and deaths.