My daughter was born in the US and she got a vitamin k injection right when she was born. Don't know if they do that in HK. Good question because I am due for baby 2 in HK in November! I am seeing the pediatrician this week so I'm going to ask!
In the US, the rationale for giving vitamin k is that newborns are born with a “deficiency” of Vitamin K making newborns more susceptible to hemorrhage in the first several days of life until vitamin k is manufactured in their systems. Opponents say it contribute to jaundice... I think some countries give it orally.