More than three-quarters of the 475,000 people aged 60-69 who got the AstraZeneca vaccine had a longer wait until July or August before getting a second dose, with some younger people fully vaccinated before them.
They had a summer of curtailed activity and now their ability to socialise this Christmas may also be limited.
It means thousands will not meet the five to six-month gap needed before becoming eligible for a booster shot.
Kingston Mills, Professor of Experimental Immunology at Trinity College, said yesterday he saw a case for expediting their booster shot in light of the Christmas socialising risk and he believed it would be safe for this age group to get vaccinated before then.
“It would be good for this group to get vaccinated before Christmas. Four, five or six months would not make a difference scientifically but it would make a big difference in terms of protection,” he added.
Earlier, chief medical officer Dr Tony Holohan said some people who had been called by the HSE for a booster shot so far were putting getting their hair done before keeping their appointment for the jab.
He said the HSE was facing a challenge and being told by some people invited for the booster “they have a hair appointment or work commitment”.
“Now is not the time to put that off,” he said.
He was speaking amid modelling forecasts that around 200,000 people could become infected with the virus next month, with 4,000 admissions to hospital and up to 450 people with the disease in intensive care over Christmas.
Dr Holohan said the number of infections could reach “even double” the 200,000 figure.
There were 634 people with the virus in hospital yesterday, an increase of 20 on Tuesday. Of these, 119 were seriously ill in intensive care, an increase of five over 24 hours.
Dr Holohan said the key measure was for people to reduce the number of people they met to reduce the risk of the virus being passed on.
Figures obtained by the Irish Independent yesterday show high numbers of people eligible for a booster shot have yet to receive one.
While the majority of nursing home residents over 65 have got a booster, only 131,200 of the 161,000 over-80s got the shot.
There are 336,000 people aged 70 to 79 and 114,200 of these have been boosted.
Among the 60 to 69 group, 22,200 have got a booster jab. Around 59,000 people who have a lowered immune system have got an additional vaccine.
The HSE is under pressure to speed up the roll-out of the boosters, which have led to a significant improvement in containing the spread of the virus among the over-80s.
At a meeting of the Fianna Fáil parliamentary party last night, Taoiseach Micheál Martin said he had asked the National Immunisation Advisory Committee (Niac) to review the five-month time frame for booster shots, especially for those who received the one-shot Johnson & Johnson vaccine.
He told TDs people needed to reduce their contacts or hospital and that intensive care admissions would be at capacity before Christmas.
Mr Martin told the gathering that everyone over 50 and those with underlying conditions would get boosters soon.
He said the country was entering a “challenging period” and there are “no guarantees” regarding restrictions
He added that antigen tests will be rolled out on a larger scale, but said Dr Holohan was still concerned about the false readings from the rapid tests.
Another 43 Covid-19 related deaths were reported last week, bringing the pandemic total to 5,609 so far.
Of the 55,272 cases of the virus reported in the last week, the source of transmission was not known in around 61pc of infections.