Reshaping Adult Pneumococcal Disease Through Paediatric Vaccination

Authors: Ioannis Passaris^a, Stéphanie Depickère^b, Chloé Abels^c, Jessica Vandenhaute^c, Raymond Vanhoof^a, Pieter-Jan Ceyssens^a

^a Bacterial Diseases Unit, Sciensano, Brussels, Belgium ^b Platform for Interventional Studies, Sciensano, Brussels, Belgium ^c MSD Belgium, Brussels, Belgium

Journal: Vaccine: X
Published: 22 May 2026
DOI: https://doi.org/10.1016/j.jvacx.2026.100835

Paediatric pneumococcal conjugate vaccination reshaped the serotype landscape of adult non-invasive pneumococcal disease in Belgium (2002–2014 and 2020–2023)

Abstract

Background

Pneumococcal disease remains a major cause of mortality worldwide in both infants and adults. In adults, vaccine uptake is often low and herd immunity limited.

Aims

The aim of the study was to analyse the impact of pneumococcal conjugate vaccine (PCV) implementation in the childhood vaccination program on the serotype distribution of adult patients with non-invasive pneumococcal disease (NIPD) in Belgium between 2002 and 2023.

Methods

We retrospectively serotyped pneumococci isolated from adults with NIPD between 2002 and 2014 (n = 2831). In addition, more recent NIPD isolates (2020–2023, n = 489) were analysed to elucidate adult NIPD serotype dynamics in the last two decades and link this with the coverage of currently available higher valency vaccines.

Results

The results demonstrate a significant decrease of PCV7 serotypes (31% absolute decrease, p < 0.00001; Cochran-Armitage trend test), as well as PCV13, PCV15, PCV20 and PPV23 serotypes, in adults with NIPD following the implementation of PCV7 in the childhood vaccination program. This effect was observed across all adult age groups illustrating the extent of herd immunity in protecting the predominantly unvaccinated adult population. However, serotype replacement was evident with non-vaccine serotypes increasing significantly in this time period (22.9% absolute increase, p < 0.0001; Cochran-Armitage trend test). Comparison with more recent NIPD data showed that serotypes included in PCV21 increased significantly (21.2% absolute increase, p < 0.00001; Cochran-Armitage trend test) in prevalence and now cause most NIPD in the adult population in Belgium.

Conclusion

In conclusion, this study elucidates the serotype dynamics of the adult NIPD population in Belgium over two decades and demonstrates how PCV implementation shaped the current serotype landscape.