Vaccines protect against serious covid infections and admissions to ICU with the virus.
That's according to Dr Augustine Pereira, the Director of Public Health North East with the HSE.
He added that there is a disproportionate number of unvaccinated patients in hospital with the virus.
Co Monaghan has the lowest vaccine uptake rate nationwide, with 87.9% of people over 18 being fully vaccinated.
Last year, the HSE said it was "aware" that residents in border counties have been vaccinated in the North.
HSE National Director, Damien McCallion claimed it was "not possible for accurate data on the numbers involved", despite the HSE "working closely with colleagues in Northern Ireland on this".
But speaking on The Wider View, Dr Augustine Pereira said health officials "have a good handle" on the vaccine figures:
