You do not need any vaccinations to visit Hong Kong, although you should ensure that have adequate medical travel insurance to cover any eventuality within Hong Kong, because hospital treatment locally can be expensive, and the cost of medical repatriation from the area is very high.
There are reports of Dengue fever in the area every year, although the risk is minimal, providing that you take precautions against Mosquito bites.
In 2003 the SARS outbreak caused the World Health Organisation to advise against travel to the area. The risk has now faded, although there have been cases since the large outbreak, and you should keep up to date with the latest news.
