The National Immunisation Advisory Centre says anecdotal evidence shows fewer babies are being exposed to viruses such as bacterial meningitis and hepatitis B as a result of human error by nurses.
In order to fully immunise babies against those viruses, an extra component must be added to the vaccine. The vaccine is given to children under five months in three stages.
But the advisory service says nurses sometimes forget to add it.
Director Nikki Turner says the helpline service receives up to three calls a month reporting the problem.
Dr Turner says the centre now has education programmes in place to help avoid further problems.