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

BACKGROUND: 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.

METHODS: 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.

RESULTS: 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.

CONCLUSION: 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.