Any attempts by the Minister for Health to make the flu vaccine mandatory for health professionals would “push it into another sphere”, Siptu says.
Paul Bell, the union's health division organiser, said such a move would be "regressive".
Mr Bell told Newstalk Breakfast there were many reasons why health care professionals might not want to get the flu vaccine such as a fear of "needle sticks" or the side effects from the vaccine.
“It’s about choice. People cannot be forced if they have a fear of injections. Where do we stop? Will the general public be forced to get the vaccine?”
With flu cases affecting hospital intake, latest The Irish Nurses and Midwives Organisation (INMO) figures on Monday reported 506 people left on trolleys in emergency departments and wards.
Waiting numbers reached a record level of 677 at the beginning of the month, illustrating the growing strain on hospitals to process patients through to beds on admission.
Minister for Health Simon Harris has said he is considering making it obligatory for healthcare professionals to have the anti-flu jab and that numbers availing of the vaccine has increased. But he added that the uptake was still too low.
“I am nowhere near satisfied that healthcare professionals, who know the danger of the flu, that their uptake rate is so low and so much lower than the population at large,” said the Minister.
“It deserves very serious consideration by me, I am not ruling it out. Mandatory vaccines is not straightforward; I prefer to do things collaboratively.”
Mr Bell said that the Minister’s comments were not helpful and claimed that workers in public transport or the retail sector were “as exposed” to the flu virus as health care workers.
The INMO has also said it was opposed to the idea and it would be forced to challenge the human rights aspect of any such move.