Increase in serotypes 4 and 14 in invasive pneumococcal disease in adults aged 18 to 59 years. A retrospective population-based descriptive study during the period 2007-2025 in the Community of Madrid, Spain.

INTRODUCTION: Vaccination programmes against invasive pneumococcal disease (IPD) for adults aged 18-59 years in the Community of Madrid (CM) are targeted exclusively at people with risk factors. The aim of the study is to describe the epidemiology of IPD in adults aged 18 to 59 years between 2007 and 2025, evaluating serotype distribution temporal changes and its implications for PCVs use in the CM. METHODS: A retrospective population-based descriptive study was conducted on clinical, epidemiological, vaccination status and microbiological information of IPD cases reported through the Notifiable Disease Surveillance System of the CM between 2007 and 2025. Incidence rates (IRs) and IR ratios (IRRs) were calculated overall, by vaccine and serotypes, using 2015-2019 as the reference period. RESULTS: Overall, 3,632 cases of IPD were reported in the population aged 18-59 years, 2,253 (62.0%) in males. Among them, 1,951 (53.7%) cases presented risk factors that made them eligible for vaccination. Vaccination was more frequent in patients with risk factors (16.7%) versus those without (3.2%). During the 2023-2025 period, IRs were below 6 cases/100,000 population. The most prevalent serotypes were 8 (24.9%), 3 (16.3%), 4 (9.4%), and 14 (6.9%). Compared to the 2015-2019 period, we observed increases in three PCV13-included serotypes in 2025: 14 (IRR: 6.7; 95% CI: 3.5 to 12.6), 9V (IRR: 6.2; 95% CI: 2.1 to 17.8), and 4 (IRR: 4.5; 95% CI: 2.5 to 7.8). Serotypes 14 and 9V were associated with penicillin non-susceptibility (PNS). CONCLUSIONS: Persistence of serotypes 8 and 3 and the notable increase in PCV20-covered serotypes 4 and 14 reinforce the importance of IPD close surveillance while PCV20 remains part of the adult immunisation schedule in the CM. The potential benefits of PCV21 in preventing disease caused by additional serotypes should also be considered.