Hospitals in Ireland suspected of being hit by the international cyber attack had been targeted by a different, older virus, health chiefs have revealed.
About 52,000 PCs and 2,350 servers in the Health Service Executive (HSE) have had anti-virus software deployed as IT experts moved to counteract the threat of the global "ransomware".
After concerns were reported about three hospitals, cyber security analysts said an older virus had been found.
"In each of these cases, the virus discovered was proven not to be the WannaCry virus, but an older virus for which protection was available.
"In all three cases, the hospital was returned to the health network and continued to deliver patient care with no impact," the HSE said.
Microsoft patches have been added to more than 28,000 new machines and more than half of the affected Microsoft XP machines have been checked and it is expected that the rest will be visited in the next 48 hours.
The HSE said another 1,300 health servers will be rebooted this evening.
The situation in Ireland is being monitored by the National Cyber Security Centre in the Department of Communications, Climate Action and Environment.
It confirmed a small centre in Co Wexford, funded by the HSE, had fallen victim to the WannaCry virus but it is not linked to wider health service IT network, preventing a wider spread of the malware.
Precautions included removing the facility from incoming email list and removing the affected hardware from the system and placing patches on remaining equipment.
Denis Naughten, Minister for Communications, Climate Action & Environment, briefed a cabinet sub-committee at Government Buildings on the issue and warned: "It is still possible that further incidences will arise and a sustained period of vigilance will be required, both in terms of updating and patching software and monitoring equipment."
The state's cyber security chiefs were also liaising with the Garda Cyber Crime Bureau and the Defence Forces.
More than 200,000 institutions and organisations were infected by the "WannaCry" virus which struck in the UK and Spain first on Friday before spreading around the world.
The NHS was one of many major global organisations affected, with 47 trusts hit.
In Ireland, HSE staff were asked to turn on computers on Monday morning but not log on for two hours to allow checks to take place and updates to anti-virus software where necessary.
External emails were also blocked to much of the HSE, including Tusla, the Child and Family Agency.
The HSE warned that any child welfare alerts that members of the public sent in over the weekend may not have been received as external emails were being rejected as a deliberate precaution.
"In many situations, Tusla relies on referrals from members of the public and organisations and individuals working with children to identify children at risk," it said.
"If anyone has reported a child protection concern to Tusla by email since Friday evening, please be advised that this may not have been received.
"We ask anyone who has reported a child protection concern by email since Friday, or who has a concern about a child, to contact their local social work duty team immediately."
Elsewhere, the HSE said it opened six helpdesks and ran a communications campaign to advise staff about turning machines on and waiting for anti-virus upgrades before starting online work.
Richard Corbridge, HSE chief information officer, said: "The team have delivered new and innovative ways of ensuring that systems are secure and are now continuing to implement these across the wider health system."
Emails into the HSE will continue to be blocked until Wednesday.