Three staff members at Altnagelvin Hospital in Derry have tested positive for Covid-19.
Another 15 members of staff are off work and isolating as a precaution amid an outbreak on Ward 40 of the hospital.
The Western Health Trust said all 23 patients on the ward have been tested and none has received a positive result.
Dr Bob Brown, director of primary care and older peoples services, for the trust said: "The trust is taking all the necessary infection prevention and control measures to protect our patients and staff and the Department of Health and Public Health Agency have been informed of the situation."
Earlier it emerged that 10 ambulance staff at Craigavon, Co Antrim have tested positive for the virus.
A further six workers have also been advised to self-isolate.
It has been reported that the 10 who tested positive had recently been involved in a golf trip to the Republic.
Michael Bloomfield, chief executive of the Northern Ireland Ambulance Service said they are in contact with the workers who tested positive and ensuring they receive support.
He said it happened almost two weeks ago and hopes to see the staff returning to work soon after completing their self-isolating periods.
Mr Bloomfield said none of their staff who tested positive for the virus became seriously ill.
"Craigavon station has approximately 60 staff so there were a total of 15 people who were on this trip, 12 of them were ambulance staff, 10 of those have tested positive, two of them have not tested positive," he told the BBC.
"It is a considerable proportion of the staff in Craigavon station.
"They're not all staff who provide the emergency ambulance response.
"These staff provide a number of roles, including our non emergency transport."
He added: "our internal review has established this was a community transmission, it was not contracted through work.
"We believe the first person we learned who tested positive on September 13 contracted that through a family member, and then as a result of a number of staff being away together for a number of days, that is how it passed.
"It just highlights how contagious this virus is and remind us all we must remain vigilant at all times."
Investigations are ongoing following outbreaks at two hospitals in the Southern Health Trust.
Six deaths have been linked with cases at the haematology ward in Craigavon Hospital.
Five patients have died following a spate of cases at the male medical ward at Daisy Hill Hospital in Newry.
On Thursday, 189 new cases of Covid-19 were confirmed in the Department of Health's daily update.
This brings the total number of infections in the north to 9,950, including 1,128 in the last seven days.
There have been no further deaths recorded by the department, with the toll remaining at 577.
