The 72-year-old woman had not travelled in the 14 days before she fell ill and was quarantined at the Lei Yue Mun Park and Holiday Village.

Appearing at a media briefing without wearing a face mask this time, Lam said major land crossings at Lo Wu and Lok Ma Chau would be closed at midnight, along with the Hong Kong-Macau Ferry Terminal – the three accounted for about 60 per cent of cross-border passenger traffic in 2018.

The Kai Tak Cruise Terminal, with far lower traffic in comparison, would remain open.

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“These measures are consistent with the initiatives rolled out earlier,” Lam said, referring to her previous decision to close six smaller border crossings that accounted for about 7 per cent of passenger traffic in 2018. “They are completely irrelevant to the strike.”

Lam stuck to her guns that large numbers of Hongkongers themselves still needed to cross the border, and barring all mainlanders from entering would be discriminatory.

“When it comes to infection control, when it comes to a virus, there is no boundary, so you cannot differentiate that people of a certain race, of a certain nationality ... are more prone to infection than other people, we have to treat them equally,” she said.

After the first six checkpoints were closed, the number of Hong Kong, mainland and other travellers entering the city had dropped by 57 per cent, 62 per cent and 49 per cent respectively, she said.

But as more than 100,000 people had still crossed the border on Sunday, the additional closures were necessary.

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That was not enough to pacify the Hospital Authority Employees Alliance, which launched the first phase of a strike on Monday to pressure the government into ordering a total border shutdown.

The Hospital Authority said about 2,700 staff had skipped work, including around 300 doctors and 900 nurses, which “seriously affected” various services, including cancer and balloon angioplasty operations which had to be delayed.

The newly formed union, emerging from recent months of anti-government protests, claimed partial victory over the government.

“The government has felt the pressure by our action and now it has agreed to shut down more border crossings. We have successfully forced the government to move forward,” alliance chairwoman Winnie Yu Wai-ming said. “But that is not enough. There are still three crossings and many mainlanders can continue coming to Hong Kong through there.”

The alliance vowed to push ahead with the second phase of the strike on Tuesday, this time predicting that as many as 9,000 – including doctors, nurses and other hospital workers – would join the industrial action following the collapse of an open dialogue with Hospital Authority chief executive Dr Tony Ko Pat-sing on Monday night.

During the 30-minute dialogue, the union could not secure a promise from Ko to push the government for further border closures or an agreement not to penalise any staff who participated in the strike.

It estimated about 5,000 of those taking part in Tuesday’s strike would be nurses, which would hit ward services hard. Up to 800 strikers each worked at major public hospitals, including Queen Elizabeth and Prince of Wales, the union said.

The Hospital Authority said it expected some essential services to be disrupted, which could entail shutting down some of the wards at major hospitals.

“As far as possible we would hope to maintain our services,” Ko said. “I would hope to urge our colleagues once again not to use a strike as a means to fight for their demands. There are many other ways to express their views.”

Brenda Cheung Fung-ching, a nurse at Pamela Youde Nethersole Eastern Hospital in Chai Wan, said about half of her colleagues at the acute medical ward where she worked would go on strike.

“Carrie Lam is just not listening to what most of Hong Kong’s public is demanding,” she said.

Lam also stressed the need to set up more quarantine camps to isolate infected people and their close contacts.

“I urge residents and district councillors not to oppose any more,” she said, referring to public opposition to such facilities being designated near residential areas.

Officials confirmed on Monday that the century-old Heritage Lodge in Lai Chi Kok would be used as a quarantine facility, despite strong objections by residents of the nearby Mei Foo Sun Chuen housing estate, who have been up in arms for days, protesting and confronting police.

“I completely understand that medical professionals have been on the forefront, I respect them, and we will do our utmost to supply them with protective gear,” Lam said, addressing the strikers’ concerns. “We agree with them on the need to reduce the cross-border flow … but at this critical juncture, I could not agree with the use of extreme means.”