The Health Service Executive has confirmed that six residents who died after becoming ill at a private nursing home in Co Donegal had contracted influenza.
The deaths occurred at the Nazareth House nursing home in Fahan within the past fortnight.
The HSE said four of the six residents who died had been vaccinated against influenza.
The institution is run by the Sisters of Nazareth religious congregation.
The 39 other residents being cared for at the home have received anti-viral treatments, with 11 of them showing flu-like symptoms.
The Health Information and Quality Authority visited the premises last night and again today.
In previous inspections, HIQA found evidence of good practice at the nursing home and a commitment among staff to continually improve the service provided.
The HSE said all appropriate treatment and control measures remain in place to ensure the outbreak is contained.
Dr Darina O'Flanagan of the Health Protection and Surveillance Centre said she is happy all appropriate measures were taken to deal with the outbreak at the nursing home.
She said proper control measures had been put in place by the northwest public health department since it was informed of the situation on Sunday.
Dr O'Flanagan said that flu is dangerous to the elderly who do not respond well to the seasonal flu vaccine.
The National Virus Reference Laboratory confirmed that those who died at the home had been infected with Influenza Type A.
The HSE said the strain circulating in the Northern Hemisphere had changed slightly and the seasonal vaccine may not totally protect against it.
Director of Public Health in the northwest Dr Peter Wright said: "People visiting older residents of the Nazareth nursing home and healthcares staff should ensure that their vaccinations for influenza are up to date."
