The 29-year-old, Yan, who was jabbed by a four-centimeter pin planted in a rear seat on double-decker bus No 2A, was sent to Pamela Youde Nethersole Eastern Hospital in Chai Wan.
She was allowed to leave after treatment.
Yan told police at 8.12am she boarded the Yiu Tung-bound bus on King's Road in Quarry Bay, sitting on the seat on the lower deck.
She soon felt a pricking sensation in her butt, after which she found the protruding pin planted in the seat.
Yan then told the bus driver to stop the bus, alerting him about the incident.
Meanwhile, she reported the case to the police.
Later, constables combed the bus, seizing CCTV footage and the pin for investigation.
Police classified the case as common assault and the eastern district crime squad was assigned to probe the incident.
Citybus and New World First Bus said the bus driver concerned had informed the company about the incident.
"Citybus and New World First Bus express concern on the matter, and will fully comply with the police inquiry," the company's spokesman said.
Similarly, on October 21, a female passenger was pricked in her waist by a 3cm pin on a double-decker Kowloon Motor Bus No 59X.
The bus was bound for Mong Kok East MTR Station. At around 9pm, the woman told the bus captain that she was injured by the pin planted in the back of the seat on the upper deck.
The bus captain then stopped the bus, reporting the case to the police and making a U-turn to Kowloon Motor Bus's Lai Chi Kok depot.
Later, police combed the bus at the depot, seizing the pin for investigation.
On January 23 last year, a 47-year-old woman was also jabbed in her waist by a 5cm pin planted in the back of a seat on a double-decker Kowloon Motor Bus No 69M.
The woman, Sung, who was injured as the bus reached Tin Yan Road, Tin Shui Wai, was sent to Tin Shui Wai Hospital conscious.
Similar incidents had occurred in 2018, which saw at least 13 Kowloon Motor Bus vehicles have their seats planted with pins within 11 days.
Police arrested a 30-year-old man in the same year but believed that he was only responsible for three of the incidents.
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