Hong Kong hospitals cut services as medical workers were striking for a second day Tuesday to demand the border with mainland China be shut completely to ward off a virus that caused its first death in the semi-autonomous territory and that authorities fear could be spreading locally.

All but two of Hong Kong’s land and sea crossings with the mainland were closed at midnight after more than 2,000 hospital workers went on strike Monday. But on Tuesday, health authorities reported two additional patients without any known travel to the virus epicenter, bringing the number of locally-transmitted cases up to four.

Chuang Shuk-kwan, who heads the communicable disease branch at the Center for Health Protection, said the growing caseload “indicates significant risk of community transmission” and could portend a “large-scale” outbreak.