The move followed days of rising pressure from health care workers, experts and even lawmakers who support Mrs. Lam’s government, and reflected distrust of the mainland as evidenced both from recent protests and the 2003 SARS crisis, in which nearly 300 people died in Hong Kong alone.

Elsewhere, officials in Germany and Japan reported the first known cases of human-to-human transmission of the virus — meaning countries now have to worry not only about quarantining infected travelers, but also about keeping the virus from spreading within their borders.

Toll: At least 106 people have died, China said on Tuesday, and the number of cases increased to 4,515 on Tuesday, from 2,835 on Monday, according to the National Health Commission. The youngest confirmed case is a 9-month-old girl in Beijing.

What’s next: China has extended the Lunar New Year holiday to Feb. 3, and some major cities have gone further, telling businesses not to open until the next week.