CHANGES IN INCIDENCE AND SEROTYPE DISTRIBUTION OF PEDIAT-RIC INVASIVE PNEUMOCOCCAL DISEASE AFTER THE INTRODUCTION OF 15-VALENT PNEUMOCOCCAL CONJUGATE VACCINE IN CATALONIA, SPAIN. A MULTICENTER SURVEILLANCE STUDY

ABSTRACT Background Serotype 3 (S3) has remained a major cause of invasive pneumococcal disease (IPD) despite its inclusion in 13-valent pneumococcal conjugate vaccine (PCV). In October 2023, a 15-valent PCV (PCV15) including S3 was introduced into the Catalan universal childhood immunization program. Methods We conducted a retrospective pre-post surveillance study to compare pediatric IPD incidence in Catalonia during a pre-PCV15 period (October 1, 2022-September 30, 2023) and two post-PCV15 periods (October 1, 2023-September 30, 2024, and October 1, 2024-September 30, 2025). All IPD episodes in children <18 years attended in 34 hospitals were included. IPD was defined as detection of S. pneumoniae in a sterile site by culture or PCR. Results 323 IPD episodes were identified in 319 children (mean age, 4.5 years). Overall IPD incidence declined from 13.0 to 9.4 episodes per 100,000 children in the first post-PCV15 period compared with the pre-PCV15 period (28% reduction; p =0.02), but returned to baseline in the second post-PCV15 period. S3-IPD incidence decreased significantly from 4.1 to 1.6 episodes per 100,000 (60% reduction; p =0.001) in the first post-PCV15 period and remained lower in the second period: 2.3 episodes per 100,000 (42% reduction compared with baseline; p =0.04). In contrast, IPD incidence caused by PCV7 serotypes increased from 0.3 in the pre-PCV15 and first post-PCV15 period to 2.7 episodes per 100,000 in the second post-PCV15 period (690% increase; p <0.001). Conclusion PCV15 introduction was associated with a sustained reduction in S3-IPD over two years. However, a marked increase in PCV7 serotypes offset overall gains in IPD incidence. SUMMARY PCV15 introduction in Catalonia achieved sustained reduction in serotype 3 invasive pneumococcal disease over two years, but a marked increase in PCV7 serotypes offset the overall disease reduction in the second post-vaccination year.