Speaking on This Week on RTÉ Radio 1, he said also that there was uncertainty surrounding the Omnicron variant and the “impact it may have.”
He added that while the “signs are encouraging” there could be no premature measurement of the situation.
The HSE is about to mark a milestone of 1m boosters being administered.
Mr McCallion said most health workers have been vaccinated with a third jab. The over-70s booster programme is two-thirds completed.
It is expected to be completed within a few weeks, he added. The booster is also being offered currently to those aged 60 and over and to those aged 16 to 59 with an underlying health condition.
Mr McCallion said: “We know people are busy with Christmas, we are trying to encourage people to take up appointments and if not possible, look at walks-ins and pharmacies.”
“Structured walk-ins” were taking place and numbers had to be controlled.
Different age groups are being identified on the HSE website for different days to maintain smaller numbers.
“The over-50s are imminent, around the middle of December,” he added.
Mr McCallion said the country is in a “very busy period” just ahead of Christmas and thus it was expected it “will take people longer to take up appointments.”
“But society is in a different place,” he added, “when the level of first appointments were higher.”
“In relation to the third dose, it's strongly recommended to those who are vulnerable,” he said.
“It’s really important people come forward for that booster, the people most exposed to people at risk of the virus.”
Regarding those who remain unvaccinated, Mr McCallion said there had been a rise in those who’d been vaccine hesitant coming forward in recent weeks for the jab.
“We are still seeing substantial numbers,” he said. Mr McCallion said 10,000 had come forward for their first dose last week and another 10,000 for the second injection.
“We are seeing an uptake of the unvaccinated,” he said. “They can receive their first dose in a pharmacy or any vaccination centre, or walk in.”
Virologist Gerald Barry earlier told Brendan O’Connor on RTÉ 1 that more testing was needed to try to control the spread of the virus due to the new Omicron variant.
He said levels needed to be at 125,000 a day, rather than 25,000 a day.
Mr McCallion said the HSE was “constantly trying to grow capacity”. “We have seen some improvement in people's ability to access a test.”
However, he added, it had to be something to “bear in mind” that there’s a point when the disease is so prevalent that people needed to be aware they had to take all actions to prevent spread.
These include restricting movements for at least 48 hours after symptoms disappear. People also needed to keep checking online, as test slots would become available, he added.
The HSE had grown testing capacity by up to 245,000 a week, he added.
“Our target by the end of the year is to be at 225,000,” he said.