A national study of 20,000 patients has identified the underlying conditions linked to more severe outcomes from Covid-19.
The conditions identified were chronic heart disease, a chronic neurological condition, chronic kidney disease and cancer.
Patients who were medically obese, with a Body Mass Index equal to or greater than 40, were also at higher risk.
The study looked at both hospital and community cases and covered the period of March to July last year during the first wave.
It was conducted by the Royal College of Surgeons in Ireland University of Medicine and Health Sciences and the Health Service Executive Health Protection Surveillance Centre.
People aged 61 can register for their vaccine today on the HSE website or by phoning the HSE helpline on 1850 24 1850.
The HSE has said that almost 25,000 people registered for a vaccination today.
The total number of registrations so far among those aged 61-69 years is 255,744, the vast majority of which have been via the HSE portal.
Registrations for people aged 60 open tomorrow.
#COVIDVaccine registration is open for people aged 61. You will be vaccinated with the AstraZeneca vaccine. The quickest & easiest way to register is online - you will need a:— HSE Ireland (@HSELive) April 26, 2021
Register here➡️https://t.co/nTNxEKexNZ pic.twitter.com/TPNDalHfPO
Meanwhile, the further easing of Covid-19 Level 5 restrictions comes into effect today.
The Government will this week consider plans for further reopening during May.
It comes as the HSE has said that more than 42,900 people have got a Covid-19 test in one of its walk-in centres since they opened. Of these 1,461 or 3.4% have tested positive.
The centres are free, no appointment necessary, and for people with no symptoms.
A new walk-in centre opened today in Castletown House, Celbridge, Co Kildare. From Wednesday, a walk-in testing service will operate in Clonminch, Tullamore, Co Offaly.
The walk-in sites in Dublin, operational for over a week in the Liberties (St Catherine's Community Sports Centre), Coolock (Parnell's GAA Club) and Mulhuddart (Cum ann Naomh Peregrine) are having their opening extended and will remain open until later this week.
In Northern Ireland, the Department of Health has today reported 73 further cases of Covid-19 and one further death.
The official death toll now stands at 2,143. The average seven-day incidence rate per 100,000 is now 36.9.
The area with the highest rate is now Mid Ulster on 74.6, while the lowest remains Ards and North Down on 11.8 per 100,000 population.