The Hong Kong government on Friday night confirmed three additional cases of novel coronavirus infection, in addition to the two confirmed earlier.

In the meantime, a leading microbiologist warned of the novel coronavirus' high attack rate of 83 percent in family members who have been in Wuhan, cautioning that the next few days could be "the last window of opportunity" to stop it from spreading in the city.

According to a government statement, the three patients, including a female and a couple, all live in Wuhan, Central China's Hubei province. All were admitted for isolation and are now in stable condition.

According to the 62-year-old female patient, she travelled from Wuhan to Shenzhen and then from Shenzhen to Hong Kong by high-speed rail on Jan 19.

After developing fever and a cough on Jan 20, she sought medical treatment at Tuen Mun Hospital's Accident and Emergency Department on Thursday and was admitted to an isolation area on Friday.

She had visited a wet market in Wuhan and a relative with a cough in a hospital there during the incubation period, but had no exposure to wild animals. Preliminary investigations revealed that she came to Hong Kong with her husband to visit their daughter and son-in-law and stayed at South Hillcrest in Tuen Mun. Her husband, daughter and son-in-law are asymptomatic and will be transferred to Lady MacLehose Holiday Village for quarantine.

The other two patients are a couple - a 62-year-old woman and a 63-year-old man. They arrived in Hong Kong by high speed rail from Wuhan at about 8 pm on Wednesday. After developing symptoms such as fever and a cough, they went to the Prince of Wales Hospital's Emergency Department on Thursday and were admitted for isolation.

According to the patients, they neither visited any health care facilities, wet or seafood markets during the incubation period, nor had any exposure to wild animals or poultry, or contact with patients with pneumonia symptoms or upper respiratory tract infection.

Preliminary investigations revealed that the couple stayed at Lake Silver in Ma On Shan with their daughter and a domestic helper. Their daughter developed fever on Thursday and will be transferred to Prince of Wales Hospital for isolation. The domestic helper is asymptomatic and will be transferred to Lady MacLehose Holiday Village for quarantine.

Meanwhile, the government is closely monitoring an overseas confirmed case of novel coronavirus infection involving a patient who boarded Cathay Dragon flight KA853 on Sunday from Wuhan to Hong Kong for transit and departed on Cathay Pacific CX500 on the same day after the onset of symptoms. Members of the public who traveled on these two flights are urged to call the hotline.

On Friday night, the president of the Center for Infection at the University of Hong Kong Ho Pak-leung quoted Yuen Kwok-yung, the city's leading epidemiologist, that the novel coronavirus shows a very high attack rate in family clusters and that it has reached 83 percent in family members who have been in Wuhan.

The next few days could be the last window of opportunity to stop the infection from spreading by people who don't know they're infected coming into the city as the incubation period is 3 to 6 days in a family cluster, Ho said in a social media post.

Apart from the 5 confirmed cases, as of noon of Jan 24, the Centre for Health Protection (CHP) of the Department of Health of HKSAR government has received reports of a total 239 suspected cases.

Among them, 122 reported cases have been ruled out as novel coronavirus infection and were discharged already. The remaining 122 cases were hospitalised.