2.2. Population, Intervention, and Comparator

In the Italian setting, three risk groups eligible for pneumococcal vaccination are stratified. The low-risk group includes healthy adults aged 65 years old who are eligible for vaccination, as recommended by the NIP. This risk group comprises approximately 53.18% of the 65-year-old cohort (N = 387,993), according to the National Survey on Health Status (Multiscopo) [16,17]. The at-risk group includes individuals aged 50 to 100 years who have one or more underlying conditions or risk factors that make them more susceptible to developing PD (e.g., chronic heart disease, chronic respiratory disease, liver disease, or diabetes mellitus). Collectively, 37.75% of adults aged 50+ years are considered at risk (N = 10,255,405). Finally, the high-risk immunosuppressed group accounts for 9.07% of individuals aged 18 to 100 years (N = 4,552,553), who suffer from underlying morbidities compromising their immune system [18].

The intervention of interest for all risk groups is the new PCV15 vaccine in combination with the currently recommended PPSV23 vaccine. The currently recommended sequential regimen of PCV13 + PPSV23 and No Vaccination are both considered as comparators for all risk groups. PCV20, both by itself and in combination with PPSV23, is not included as a comparator for the high-risk group due to a lack of clinical studies investigating PCV20 in immunocompromised patients [19].