The AstraZeneca Covid-19 vaccine should not be administered to people under the age of 60 in the Republic of Ireland, the National Immunisation Advisory Committee says.
The panel recommended restrictions on its use after the European Medicines Agency warned that rare blood clots have been associated with the jab.
In a statement, NIAC said the AstraZeneca vaccine "is not recommended for those aged under 60 years including those with medical conditions with very high or high risk of severe Covid-19 disease".
The advisory body added that the AstraZeneca vaccine should be limited to those aged 60 and older.
NIAC chairwoman Professor Karina Butler said the committee realises the need to balance the significant benefits of a national vaccination programme with the very rare risk of reported blood clot events.
"While this is an extremely rare condition, consideration must be given to the fact that it has a very high risk of death or severe outcome," she said.
"As the risk/benefits of Vaxzevria Covid-19 AstraZeneca vaccine may vary by age and as alternative Covid-19 vaccines are available, NIAC has revised the recommendations for use of this vaccine.
"In developing these recommendations, NIAC is informed by the available scientific evidence and the risk/benefits of the vaccines. New evidence will be reviewed once available and any further required recommendations will be notified to the Department of Health."
Prof Butler told a press briefing at the Department of Health the rare blood clotting is estimated to occur in between four and 10 in every million people, one of whom may die.
"However, as so few events have been reported, there is a high level of uncertainty regarding the incidence of this extremely rare adverse event in any particular age group, or gender," she said.
"Although most cases occurred in women under 60 years of age. This may be because of the higher rates of vaccination and healthcare workers who are predominantly female."
NIAC said anyone under 60 who has had their first dose of the AstraZeneca vaccine with a "very high-risk or high-risk medical condition" should receive their second dose 12 weeks later as scheduled.
But anyone under 60 who is not considered to be high risk should have the second dose extended to 16 weeks "to allow for further assessment of the benefits and risks as more evidence becomes available".
Anyone aged 60 and older should receive their second dose 12 weeks later as scheduled.
The HSE said it had written to hospital groups and community healthcare organisations on Monday evening to advise them that all AstraZeneca clinics planned for Tuesday "should be cancelled in light of updated guidance received from the National Immunisation Advisory Committee, and the Department of Health".
"Anybody due to attend an AstraZeneca clinic is therefore advised not to do so," the HSE said.
It added it will be in contact with patients in due course to rearrange appointments.
It has raised concerns that any restrictions could affect the country's vaccine programme.
Deputy chief medical officer Dr Ronan Glynn said the change does not necessarily mean there will be a delay to the vaccination programme.
"It will likely have an impact - the extent of that impact, though, remains to be seen," he said.
"It's not necessarily the case that this will have a material impact or delay on the rollout of the programme at a population level."
He said those who had received the AstraZeneca jab to date should be "reassured that they've got a very effective vaccine".
"I would hope that if they can see that for any age group the risks associated with receipt of this vaccine are very, very small," he added.
NIAC's advice came as Ireland began to lift some of its level 5 restrictions.
Monday saw the 5km travel limit relaxed to allow people to travel anywhere within their county, or up to 20km if crossing into another county.
People from two households will be allowed to meet outdoors for social and recreational purposes, but the Government has insisted that private gardens remain off limits.
All schoolchildren are to return to in-class learning from Monday, while construction on housing and essential projects will also resume.
Further easing of restrictions is expected on April 19 and 26.
Monday saw a further 394 cases of Covid-19 confirmed in Ireland. No further deaths linked to the virus were recorded.
The Irish Department of Health said as of 8am there were 227 people with the disease in hospital, 50 of whom were in intensive care.
The five-day moving average of new cases is now 404, while the country's 14-day incidence rate per 100,000 population stands at 131.9.