Officials say there have been at least eight confirmed cases, with one test pending, in West Nyack and New City.
The patients are mainly among young adults in their 20s, who were connected to the party on June 17.
Officials say the party host was already symptomatic, and since the initial party, there were at least two other parties with some common attendees.
Some are said to be refusing to cooperate with contact tracers and are facing fines.
Subpoenas were issued to nine people, officials said, with $2,000 per day fines possible for non-compliance with the Rockland County Department of Health.
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