The Department of Health (DH) said the male patient, 51, was vaccinated on Wednesday last week and developed a cough and shortness of breath three days later.
On Monday, he went to the Accident and Emergency Department of Queen Elizabeth Hospital, where he remains in a critical condition.
Doctors initially diagnosed him as having suffered a heart attack.
The department said according to the Hospital Authority, the man has a history of various medical conditions including diabetes and hypertension.
Officials said the case will be referred to a panel of experts, and again reminded people that patients with uncontrolled chronic illnesses should seek their doctor’s advice before getting vaccinated.
The expert panel had previously looked into a number of cases, including three deaths, but has yet to establish any link between the mainland-produced vaccine, and any severe side effects.
A spokesman for the department said with more people getting inoculated as part of the government’s vaccination programme, reports of people who fall ill might also increase.
While he said officials will carefully monitor ‘adverse events’ following inoculations, he noted that people die every day from conditions such heart disease and diabetes.
Experts have called on the government to provide more data to the public about potential side effects associated with the Sinovac product, saying there is currently a dearth of information available.
However, the government has assured people that the vaccine is safe, and that experts here have recommended its use for older patients, even though the product is not used on over-60s in the mainland.