A further 21 employees are currently self-isolating “as a precautionary measure”, Strabane-based sportswear firm O’Neills said.
The company said it was liaising closely with the Public Health Agency (PHA) to monitor the situation.
Part of its production unit has now been closed for a deep clean.
“The health and safety of our employees and their families together with our clients and customers is our highest priority,” a company spokesman said.
“In addition to the deep clean which is currently being carried out, we will continue implementing the rigorous protocols introduced from the outset”.
Arrangements have been made for staff to work from home where possible pending the re-opening of the premises, the spokesman added.
O’Neills is the biggest private sector employer in Strabane.
Half its workforce come from across the border in Co Donegal.
There are concerns about the rising number of cases of coronavirus on both sides of the border in the north west.
On Wednesday, Holy Cross College in Strabane closed after three positive cases were recorded in the “school community” since last weekend.
New restrictions come into force in Donegal from midnight on Friday.