About one million people are currently eligible for a booster vaccine dose, according to a Prime Time analysis of HSE vaccine data.
According to Government data made available for the first time on Wednesday, 389,000 booster doses had been administered to eligible people as of Tuesday.
On Tuesday, Minister for Health Stephen Donnelly extended the rollout of boosters to include people aged 50 to 59. People over the age of 16 with an underlying health condition, and all residents in long-term care facilities, are also now eligible.
Minister Donnelly said he was announcing the decision based on advice from the National Immunisation Advisory Committee (NIAC).