Both of them are in intensive care at Queen Mary Hospital, the Centre for Health Protection said, adding the 17-month old girl had a fever and started coughing on Friday.
She is not vaccinated and did not travel during the incubation period, but two people who live with her tested positive for the virus after taking tests.
Meanwhile, officials said a 13-year-old girl was admitted to hospital after experiencing shortness of breath on Friday.
The girl, suffering from underlying illness, is vaccinated, but two of her family members tested positive for Covid.
“The centre reminded the public who have not received the initial dose of the COVID-19 vaccine, including infants and children, that they should get vaccinated as soon as possible,” it said in a statement.
The elderly and other high-risk groups, such as those with chronic illnesses, should get a booster, the centre added.
