Pregnant women and people over 60 will begin receiving their second booster vaccination against Covid-19 from today.
The Health Service Executive has urged those eligible to get the vaccine.
Since last week, people who are over 60 and women who are pregnant have been able to book their appointments through the HSE.
The injections are being administered at vaccination centres and at participating pharmacies around the country.
HSE Chief Clinical Officer Dr Colm Henry has urged those eligible to take up the offer.
He warned that there was a waning of immunity against infection from Covid-19 over time.
Dr Henry added that the booster injection would provide recipients with ongoing protection from serious illness, and increase their immunity against infection from Covid-19.
People aged between 50 and 64, as well as people with long-term health conditions, will also be offered booster vaccines in the coming weeks.
Dr Clíona Murphy, Clinical Director of the HSE's National Women and Infants Health Programme and a member of the National Immunisation Advisory Committee, has encouraged pregnant women to take up the booster, for themselves as well as their babies.
Dr Murphy told RTÉ's News at One that pregnant women were more vulnerable to the adverse affects of Covid and said that the best time to get the booster was in the later stage of pregnancy.
She added that the booster benefits included a reduction in the chances of being unwell or going to hospital but it also helps with development of antibodies.
Children younger than six months are at higher risk from Covid, Dr Murphy said, but antibodies are transferred through blood or breast milk.
People advised to book appointments
The President of the Irish Pharmacy Union has said that it is prudent that people book an appointment in advance of attending a pharmacy.
Dermot Twomey, who runs a pharmacy in Cloyne, Co Cork, told RTÉ's Morning Ireland that pharmacies can have capacity issues "and also have other appointments and prescriptions and things to do as well".
He said that he expects that vaccine take-up will increase from when the summer ends.
"Generally when August turns into September, October, people start thinking about their flu vaccination and protection for the winter," he said.
He added that a lot of people were infected with Covid-19 in recent months and they will need to wait before they can get their next vaccine.
"People have to wait at least four months after either getting their previous booster dose or if they have had Covid themselves in the previous four months, they need to wait in order to get the vaccine."
Mr Twomey also said that over the next month people in younger age groups will be able to get a second booster.
"From today it's individuals aged 60 or over and those women who are pregnant from 16 weeks on. From next week, 22 August, it will be individuals 55 and over. The following Monday 29 August - individuals 50 and over," he said.