A karaoke party at a house on the outskirts of Limavady is understood to be the cause of a COVID-19 outbreak.
It is believed partygoers followed social distancing guidelines.
But it is thought the virus was transmitted by the microphone being passed around at the house party.
Health officials are now conducting a test and trace programme to establish how many other people have the virus.
There are no plans as yet to impose a lockdown in the Co Derry town like what has happened in Leicester after a spike in cases.
The Public Health Agency has placed a mobile testing unit at the North West Regional College in Limavady.
Sixteen new cases of the virus were confirmed yesterday – nine of them were in the Causeway Coast and Glens council area, where Limavady is located.
Four were in Derry and Strabane, with one in Fermanagh & Omagh, and two in Newry and Mourne.
Anyone with symptoms of COVID-19 – a raised temperature, a continuous new cough or a change in their sense of taste or smell – can book a test online at www.pha.site/cvtesting
There are additional test sites at the City of Derry Rugby Club, SSE Arena in Belfast, Craigavon MOT Centre and St. Angelo Airport in Enniskillen.
The cluster outbreak in Limavady is clear evidence the virus is still in existence in the North of Ireland.
Health chiefs advice is to maintain a 2 metre social distance and wash hands as often as possible to prevent the spread of the virus to others.
