Health authorities have reassured parents that a five-in-one vaccine for babies, which contains traces of a mercury-based preservative, is safe and does not pose any health risk.
This follows the British government's decision to introduce a similar vaccine which, it says, does not contain any thiomersal, a mercury-containing substance used to prevent bacterial contamination.
The Irish Medicines Board (IMB) said there had been no thiomersal used in the national primary childhood immunisation programme since 1996. But it confirmed that it is present in "trace form" in a newer four-in-one vaccine, Tetravac, and a five-in-one vaccine, Pentavac.
The amounts of mercury are so small they are undetectable, and should be considered equivalent to thiomersal-free products, the IMB said.
An IMB spokeswoman said: "It is there in trace form, but it might as well not be there . . . for instance, you would get more mercury eating an orange."
The board has said, however, that hypersensitivity reactions can occur in "certain sensitised individuals" despite the low concentration. But it said research had shown there was no evidence of such reactions due to thiomerosal in other countries where both vaccines are administered.
Pentavac protects against diptheria, tetanus, whooping cough, polio and Haemophilus influenza type b (Hib). Tetravac protects against diphtheria, tetanus, whooping cough and polio.
The British government is introducing a different five-in-one vaccine, Pediacel, which will be given to infants and will replace its existing combination inoculations.
These include a whooping cough vaccine, which uses thiomersal, along with an oral polio vaccine.
It says it is moving to introduce the new five-in-one because it will eliminate any mercury and allow for the administration of an "inactivated" polio vaccine.
The British government said the elimination of the old thiomersal-containing whooping cough vaccine was "consistent with a move to avoid environmental exposure to mercury". The replacement will also contain fewer antigens - elements which provoke an immune response.
The move comes weeks after researchers in the US suggested thiomersal could be linked to the development of autism.
However, the Government said no such link has been found by other British, US and Danish studies.
British medical authorities were unable to say yesterday if the new five-in-one vaccine, Pediacel, contained any trace elements of thiomersal. The vaccine is not licensed for use in Ireland.
Independent MEP Ms Kathy Sinnott, who has campaigned for safer immunisation policies, expressed concern yesterday over any links between vaccines and thiomersal.
She said that while she was not against immunisation, it was time for health authorities to be honest about vaccines, and screen children who may be vulnerable to adverse reactions.
