At least three quarters of the Health Service Executive's IT servers have been decrypted and 70% of computer devices are back in use, following last month's cyber attack.

The HSE was forced to shut down all of its IT systems following the "significant" ransomware attack, which focused on accessing data stored on central servers.

CEO Paul Reid told the Oireachtas Committee on Health that it will likely take months before the system is fully restored.

Mr Reid said the cost of the cyber attack is likely to exceed €100 million in the short to medium term and is likely to rise significantly in the long term.

He said the HSE will establish a cyber security operations centre to monitor the HSE network, which will be set up in the near future and added that a full procurement process for the facility also getting under way this year.

Sinn Féin's David Cullinane insisted that the costs associated with the cyber-attack should not come from funds that were already allocated to the organisation to provide for the likes of extra acute hospital and community beds, more staff and ICU capacity.

Paul Reid said the HSE was focused on ensuring that secured funding is contained and ringfenced.

The committee has also heard that there will be less 80,000 Jansson vaccines delivered in June, close to the worst-case scenario.

Damien McCallion, National Lead for the Covid-19 Vaccination Programme, said 470,000 vaccine doses were expected for June.

Minister for Health Stephen Donnelly told the Dáil last month that the best-case scenario was that there would be 235,000 Johnson & Johnson Janssen vaccines in June, and the worst-case scenario was that there would be 60,000.

It's expected that there will be little more than 70,000 Janssen vaccines delivered in July.

However, given current NIAC recommendations, demand for Janssen vaccines is likely to fall significantly.

Mr Reid told the committee that 3.7 million Covid-19 vaccine doses have now been administered.

1,400 vaccinators are in place, with thousands more being recruited, and a new vaccination centre is opening in Wexford this week.

The committee also heard that Ireland is at peak vaccine supply, with over 340,000 vaccines administered last week and 300,000 planned for this week.

Mr Reid said there is a good supply line of Pfizer and Moderna vaccines for the next two weeks, meaning there will be "another couple of weeks of over 300,000" vaccines administered.