The HSE is urging people to not hold off getting a second vaccine dose, amid concerns some people are waiting to see if they're offered a different jab at a later date.
Chief Clinical Officer Dr Colm Henry says the "best bet" anyone can make is to take the second dose they're offered now.
The National Immunisation Advisory Committee (NIAC) is currently looking at proposals around mixing vaccines, to see if it could benefit the vaccine rollout in Ireland.
That would mean someone who got AstraZeneca as their first dose could get a second dose of Pfizer or Moderna.
The approach is already in place in countries such as Germany, where Chancellor Angela Merkel received doses of two different vaccines (AstraZeneca and Moderna).
However, there are now rising concerns around the Delta variant - and experts are warning that people aren't fully protected until they've been fully vaccinated.
On The Pat Kenny Show, Dr Henry said there is "rapidly" evolving evidence around mixing vaccines, with international research ongoing.
He says studies so far have shown no major concerns around potential side effects.
There's also evidence of "good, enhanced antibody response" in people who've received two different jabs.
However, he stressed it's vital people here get fully vaccinated as soon as possible rather than waiting to see if the policy around mixing vaccines changes.
He said: “At this point in time, I would urge anyone to not bet on waiting… the best bet you can make is to take that second vaccine.
“In light of this expected surge in Delta cases, you are better off fully protected with a full dose schedule from the vaccine you started with.”
He noted that evidence shows just one dose "leaves you halfway up a ladder without full protection".
Yesterday, the Tánaiste said officials are looking at whether extra AstraZeneca doses could be given to younger cohorts later this summer.
The vaccine is currently only recommended for people aged 50 and over in Ireland, amid concerns around very rare blood clotting cases.
However, Ireland could soon find itself with hundreds of thousands of extra doses once older age groups are fully vaccinated.
Dr Henry says NIAC will look at whether it'll be beneficial to give younger people the AstraZeneca vaccine as well.
He explained: “We anticipate in late July and August we will have deliveries in excess of what we need. What if we could give those to younger cohorts, and what would be the impact of that in terms of accelerating the programme.
“That’s a question that’s gone to NIAC - and of course NIAC have to weigh up the relative benefits and risks of this rare reported side effect for those vaccines - which has been well flagged - versus the emerging risk of the Delta variant.”
