The Jockey Club said on Sunday that it had closed six off-course betting branches, after people infected with Covid were found at a private gathering involving more than 40 staff.
The club said eight employees gave positive rapid test results on Thursday and Friday before reporting for duty and other staff, who attended the gathering on August 1, were then asked to take PCR tests.
It said five other workers were subsequently confirmed to have the coronavirus.
The club said the affected branches, which have been shut for deep cleansing since Friday, will remain closed until further notice.