Background and Objective:
Hexavalent vaccination (HV) is a priority for protecting infants against six potentially deadly infectious diseases. In Italy, Europe and recently in the U.S. there are three commercially authorised vaccines. HV is currently used in a primary immunisation regimen with three doses (3-5-11 months). A retrospective clinical study was conducted in Sicily, Italys fourth-most populated region (10% of national population and 9% of all newborns), in order to evaluate real life data on HV during 2016-2019.

Methods:
Data on the completion of the administration of HV, on interchangeability between the two vaccines that alternated between 2016 and 2019 (Infarix Hexa ®- Vaxelis ®), on use above the established age and on co-administration with Rotavirus and Pneumococcus vaccines were investiganted in five Local Health Unit of the Sicilian Region. Scientific coordination was in charge to the Clinical Epidemiology Unit of the University Hospital (UH) of Palermo, through a questionnaire approved by the ethics committee of the same UH.

Results:
Data collected from the LHUs of Agrigento, Catania, Palermo, Ragusa and Trapani, constituting 72.5% of the Sicilian population, showed an average of 91.5% completion of the HV cycle at 24 months of age. The average age of administration gradually increased in children who switched between the two vaccines compared to those who completed the vaccination cycle with the same product. Interchangeability with one or two doses of hexavalent was documented in 17.8% (2018) and 16% (2019) of infants. Co-administration was 93% with the 13-valent conjugate anti-pneumococcal vaccine and 70% with the anti-rotavirus vaccine.

Conclusions:
This retrospective analysis could contribute to demonstrate safety and sustainability of the interchangeability between different types of the same HV, helping Public Health Authorities to manage potential disruptions due to missed routine immunization opportunities pandemic-related or change of vaccines due to National or Regional tender.