Nl's vaccination programme is facing a delay after it was announced that under-40s are to be offered an alternative to the AstraZeneca jab.
The Department of Health has said people aged under 40 in NI will be offered the Pfizer-BioNTech jab.
Health officials have urged people to keep coming forward for their Covid-19 vaccine.
The change is not a result of new information on the vaccine's link to rare blood clots.
This latest decision is because the risk from Covid-19 is falling, which alters the risk/benefit calculations for younger age groups.
While this policy currently applies to those under 30, the Joint Committee on Vaccination and Immunisation (JCVI) has decided to move the age threshold after new and slightly increased figures on clots were reported.
The head of NI's vaccine programme, Patricia Donnelly, said that the rollout will be slower, as a result.
"This has pushed the programme back by several weeks," she told BBC Radio Ulster's Evening Extra programme.
"From time to time, as people booked on, they would find they would have to wait a few days or a few weeks to get the appointments they want," she said.
"That's going to be a feature over the next few weeks and potentially the next six, eight weeks because we will have steady amounts of vaccine but not enough to do all those people who want it when they want it.
"It will be there eventually but it just means they wait a bit longer."
Northern Ireland is ahead of other parts of the UK in offering vaccinations to people younger than 40.
Almost 20% of those aged 18-29, and almost 43% of those aged between 30-39, have already been vaccinated.
Nearly one million people here have already received their first vaccine dose, and figures indicate people are still keen to be vaccinated.
People who have already received their first jab of the Oxford-AstraZeneca vaccine will get their second jab with the same vaccine.
'Extremely rare risk'
The UK's medicines safety regulator said there had been 242 clotting cases from 28.5 million doses of the vaccine.
But it said the risk was slightly higher in younger age groups.
Low levels of coronavirus in the country and the supply of alternative vaccines has also led to the decision being taken.
That means people who are under 40 and have yet to receive their first dose will be offered an alternative to the AstraZeneca jab.
How will this affect the vaccine rollout?
The change means some logistical alterations to Northern Ireland's vaccination programme.
From Monday 10 May:
- People over the age of 40 can book AstraZeneca first-dose appointments at Belfast's SSE Arena vaccination centre and community pharmacies
- People aged 30-39 can book their Pfizer first dose appointments at the other regional trust vaccination centres
- People aged 30-39 can opt to receive the AstraZeneca vaccine from participating community pharmacies if they would rather not wait to receive an alternative to the AstraZeneca vaccine
- Anyone younger than 40 already booked for their first vaccine at one of the trust centres, including the SSE Arena, will have this appointment honoured - with the Pfizer vaccine
The move may affect the UK's target of offering a first vaccine dose to every adult by the end of July.
Northern Ireland is currently ahead of schedule on its vaccine rollout.
While this would be a setback its understood officials are looking at what is available, and further supplies are expected to arrive in Northern Ireland in the next month.
Chief Medical Officer Dr Michael McBride said: "The potential risk associated with the AZ first dose is extremely rare and the threat from Covid-19 is much higher for the majority of adults.
"Getting vaccinated against this virus gives us hope - it protects us and helps us to start reclaim normality."
He added that he was looking forward to getting his second dose of the AstraZeneca jab and encouraged everyone to "come forward without delay for their first and second jabs when it's their turn".
The Department of Health said the AstraZeneca vaccine was still "essential in the successful roll-out of our vaccination programme.
"Thanks to this programme, close to a million people here have already received their first vaccine dose, helping society to emerge carefully from lockdown."
The benefits of the Oxford-AstraZeneca vaccine still outweigh the risks for the vast majority of adults, said a spokesperson for JCVI and the health regulator the MHRA.
'Appeal for public patience'
The head of the NI vaccination programme, Patricia Donnelly, said: "Protecting our adult population through vaccination is a huge and unprecedented undertaking.
"Logistical challenges are inevitable, but the programme has already proved itself to be highly resilient.
"I would again appeal for patience from the public, as we reset the programme in light of the updated JCVI advice.
"Pfizer supplies remain steady but limited, so our progress with the 30-39 age group will be limited for the next few weeks.
"Likewise, those under 30 will have to wait a few weeks before being offered appointments for their first dose."