The boy's family of five, including his grandfather, were found to have been infected on a day when authorities reported 7,938 infections and 11 deaths yesterday.

The Hospital Authority's chief manager, Sara Ho Yuen-ha, said the boy has epilepsy and had taken his second Sinovac shot in March.

He tested positive on Friday and attended a Covid clinic on Sunday, when doctors found him suffering from shortness of breath and referred him to Princess Margaret Hospital in Kwai Chung.

He required high concentration oxygen and is in a critical condition.

"His father, mother and six-year-old sister were infected a couple of days before," Ho said. "His grandfather, who could be their source, was also infected."

Government adviser Lau Yu-lung said children with epilepsy have worse breathing and swallowing functions, and are more prone to croup if their breathing tubes are stimulated. He said kids with chronic diseases should take four jabs.

He said the effects of vaccination in children with kidney issues and cancer, who usually require immunity-suppressive medication, are not as good as healthy children.

His comments came as HKU's pediatric clinical assistant professor, Jaime S Rosa Duque, said vaccinations have helped reduced 80 percent of hospitalization in teens and children.

The 11 fatalities reported yesterday were 10 men and a woman aged 61 to 93, all of whom had chronic diseases and cancer. Ten of them were not triple-vaccinated. Five had not received any jabs at all.

The death toll in the fifth wave is 9,597.

The 7,938 fresh cases yesterday were 7,772 locally transmitted and 166 imported.

Although the daily caseload dropped below 10,000 for the third consecutive day yesterday, the Centre for Health Protection's head of communicable disease branch, Chuang Shuk-kwan, said it could be a result of the Mid-Autumn Festival long weekend.

Meanwhile, HKU infectious disease expert Ho Pak-leung said compulsory tests and building lockdowns should be axed to beat increasing anti-epidemic fatigue.