Rapid increase in serotype 4 invasive pneumococcal disease among vulnerable young male adults in Belgium (2020-2024).

OBJECTIVES

Invasive pneumococcal disease (IPD) remains a significant global health threat. Although pneumococcal conjugate vaccines have considerably reduced the burden of IPD after their implementation, changes in serotype distribution do occur. This study examines the epidemiology of serotype 4 IPD in Belgium from 2007 to 2024, focusing on a recent and pronounced increase in cases. The aim is to identify and characterize high-risk populations.

METHODS

Epidemiologic data from the passive laboratory-based IPD surveillance, as conducted by the national reference center for invasive pneumococci at UZ Leuven, were used to study the evolution of serotype 4 IPD over time. Because clinical and behavioral data are lacking in the nationwide surveillance, this was complemented with a multicenter sub-analysis for 248 serotype 4 cases (2020 to mid-2024), which represented 61% of all serotype 4 IPD cases within that period. Patient records were retrospectively consulted, and information was captured in REDCap.

RESULTS

A notable resurgence of serotype 4 IPD cases was noticed, beginning in 2020 and peaked in 2023 as the third most common serotype causing IPD in Belgium. This increase was primarily seen among young adult males (18-49 years old) residing in large urban areas (Brussels Capital Region, Antwerp, Liège, Namur, and Hainaut). The sub-analysis revealed that a high proportion of these patients were unemployed (60.9%), experienced homelessness or unstable housing (30.2%), and reported substance use (72.6%). Tobacco (63.3%) and heavy alcohol consumption (35.1%) were the most reported, whereas a subgroup used hard drugs (26.6%).

CONCLUSION

This study identifies a specific population of vulnerable adults at risk for serotype 4 IPD, emphasizing the need for a specific prevention strategy. Vaccination of this risk group with a pneumococcal vaccine targeting serotype 4 is needed to reduce the increasing burden of invasive disease caused by this vaccine serotype.