A real-time register-based surveillance system for non-invasive and invasive pneumococcal disease

While surveillance of invasive pneumococcal disease is well-established in Denmark, monitoring of non-invasive infections is not covered by existing surveillance. We aimed to expand the weekly updated Danish register-based surveillance system for severe acute respiratory infections (SARI) to cover admissions related to Streptococcus pneumoniae and to investigate the impact of invasive and non-invasive pneumococcal disease during ten years in Denmark from 2015 to 2025.

We defined 'pneumococcal SARI' as a patient identified in the SARI surveillance system with a relevant non-invasive or invasive microbiological diagnostic test positive for pneumococci. We examined baseline characteristics and severity indicators including length of hospital stay, provision of intensive care treatment and 30-day all-cause mortality, stratified by type of disease. Further, we investigated overall trends and serotype development during the study period.

Between 2015 and 2025, we identified 12,185 and 3664 patients with non-invasive and invasive pneumococcal SARI, respectively. Compared with invasive pneumococcal SARI, patients with non-invasive pneumococcal SARI had overall shorter admissions (range 2.5-5.0 days compared with 3.1-7.0 days) and a lower 30-day mortality (range 3.0-13.5% compared with 5.0-21.2% for adults aged ≥50 years). Seasonality was disrupted during the COVID-19 pandemic but restored to pre-pandemic incidence and timing during the post-pandemic years.

We have built a new system for weekly updated national surveillance that captures both non-invasive and invasive pneumococcal SARI. The system makes it possible to evaluate severity and characteristics of patients and allows for a near real-time monitoring of trends in disease burden of several respiratory pathogens.