The Victoria Harbour in Hong Kong is seen in this June 7, 2017 photo. [Photo/VCG]

The number of visitor arrivals to Hong Kong has plunged 49.6 percent year-on-year during the period of Aug 15 to 20, as 31 countries issued travel warnings for Hong Kong, according to Secretary for Commerce and Economic Development of Hong Kong SAR Edward Yau Tang-wah.