There were 1,689 people hospitalised in Ireland with the RSV respiratory virus between September and November, more than double the number of cases in the pre-Covid era.
New data from the HSE shows hospitals across Munster have seen an increase in the number of children and over-65s needing treatment for the virus this winter.
A steep jump in RSV cases saw hospitalisations rise from 129 in September, to 753 in October, before reaching 807 last month.
Between September and November of 2019, before Covid arrived, there were just 596 cases in total in Ireland. In fact, there were more cases last month than at the peak of the infection wave during the entire winter of 2019-2020.
Doctors stress that the vast majority of children experience a mild illness, however these figures show that the number of more serious cases is increasing.
The increase seems all the more startling as RSV almost disappeared during the pandemic due to social distancing and lockdowns.
Between September 2020 and February of last year, there were just two RSV patients hospitalised in Ireland; one toddler in October 2020 and one toddler in February 2021.
However, in autumn last year as normal life resumed, viruses returned. By September there were 136 cases, including 127 children under four.
Cases peaked in October at 806 cases, but November was also very high at 747 cases, dropping down sharply to just 286 by December of last year.
In contrast, one hospital, Our Lady of Lourdes Hospital in Drogheda, admitted 65 young children with RSV this October alone.
That month, between Cork University Hospital (41), the Mercy hospital (16), and the Bon Secours Cork (13) there were 70 under-fours hospitalised in Cork. This dropped to 44 last month, even as numbers rose in other areas.
In November, CHI at Crumlin admitted 50 under-fours, up from five in September but only slightly more than Portiuncula hospital with 43 sick children.
University Hospital Kerry admitted 38 children aged under four with RSV between September and November this year, compared to just 10 during the same period in 2019.
Tipperary University Hospital in Clonmel and University Hospital Waterford also treated very small numbers of young children with RSV.
The data was released by the HSE in response to a parliamentary question from East Cork Labour TD Seán Sherlock.
“It is important that information on numbers requiring hospitalisation from RSV is in the public domain,” he said.
"It is vital that the HSE maintains a campaign of information to encourage the prevention of excess hospitalisations over the coming weeks and months."
The data also shows RSV cases among over-65s.
Last month, 178 people in this age group were admitted to hospitals, meaning that there were more elderly people seriously ill with RSV than in any month over the four years covered by the data.
It is significantly higher than in November 2019, when just 22 older people were this ill.
Among the hospitals treating these 178 patients, the Bon Secours hospital in Cork and Connolly Hospital in Dublin saw the highest numbers, at 10 patients each.
Mallow Hospital treated four over-65s last month, and two patients aged between five and 64.
Information on treating childhood respiratory illness is available on the HSE website.