The HSE has urged people to come forward for booster vaccines when called, amid concerns about high no-show rates.
At a briefing on Thursday afternoon, health officials said they were seeing 50% no-show rates in some areas.
While around 1,500 people a day are getting vaccinated, according to HSE figures, there are concerns that eligible people are not coming forward fast enough for booster jabs.
The Government is placing faith in a successful booster campaign, as the fragile health system tries to cope with a dangerous surge in Covid-19 cases.
Health officials accepted it may take until next spring for some groups to receive a booster vaccine, although they stressed this was largely down to the need to leave a five-month period between an individual receiving a full dose and the extra shot.
HSE chief executive Paul Reid said: “We have seen some no-show rates in some centres varying from 25% to 50%.
“And maybe just due to a sense of security among people feeling they have had two vaccines and don’t need a booster. But we do know it’s really important for people to come forward to for it," he said.
This week, the Government announced a set of measures – including instructing the hospitality sector to follow a midnight closing time – to reverse the rising number of cases.
The fourth wave of the virus is creating major challenges for hospitals, despite the country having one of the highest vaccine uptake rates in Europe.
Damien McCallion, national director of the HSE’s Covid Vaccination Programme, said the booster programme for some eligible groups may not be completed until March.
“There will be large numbers that need to be done in the coming weeks. But many of those then will still have people who will still be eligible and need their booster right through until next March.”
It is expected the vast majority of people aged 70-79 will have at least received an appointment, if not a booster jab, by the end of November.
Among people aged 60-69, the target is the end of December.
“The five months sort of dictates nearly the duration of the booster programme, in the sense, if someone’s vaccinated today, they’ll be entitled to a booster in five months’ time,” Mr McCallion said.
“There will be, I suppose, a large number initially over the coming months, and then there’ll be smaller numbers from there.”