Do I need a transit visa if my transit time at the airport is less than 5 hours?

Yes Rehana, you need to apply for a transit entry permit. You can send your application documents directly to the HK Immigration Department.

The application documents include: application form (ID 1003A) dowloaded from the department's official website, your recent photograph affixed on page 1 of the application form, photocopy of the your passpot, photocopy of your financial status proof, e.g. bank statements, savings accounts passbooks, tax receipts, etc. and proof of employment (if any), e.g. company leave letter, salary slips, etc., photocopy of flight tickets, and photocopy of proof of the proposed visit in the US, itinerary, etc.

Post address:
Receipt and Despatch Sub-unit
Hong Kong Immigration Department
2/F, Immigration Tower
7 Gloucester Road
Wan Chai, Hong Kong