Monitoring and preventing severe acute respiratory infections: a focus on COVID-19 and pneumococcal disease

Respiratory infections present a significant public health challenge due to their high prevalence and societal impact, including substantial costs, strain on healthcare systems, and elevated morbidity and mortality rates. The emergence of SARS-CoV-2 in 2019 underscored the disruptive potential of respiratory infections, highlighting the need for continuous monitoring and preventive measures like vaccination. This thesis focuses on the surveillance and prevention of severe acute respiratory infections (SARI), particularly through vaccination strategies targeting COVID-19 and pneumococcal disease.

The initial section explores SARI surveillance methods, evaluating the utility of Diagnosis-Treatment Combination (DBC) codes, used for hospital billing, as a tool for monitoring hospital admissions. Comparing DBC codes with ICD-10 diagnostic codes from 2016-2020, the research indicates that DBC codes effectively track trends in SARI cases, aligning with seasonal patterns of respiratory viruses like RSV and influenza. While DBC codes show promise for SARI monitoring, integration with microbiological data is recommended to enhance accuracy, alongside further validation and improvements in data extraction processes.

A significant portion of the thesis evaluates the effectiveness of COVID-19 and pneumococcal vaccination programs. Studies on COVID-19 vaccine effectiveness against hospitalizations reveal high levels of protection during the early stages of vaccine rollout (70% after one dose, 93% after full vaccination against the Alpha variant), and sustained effectiveness against the Delta variant (78% after two doses, 89% after a booster). However, vaccine effectiveness was slightly lower in older adults and those with specific underlying conditions. These findings support ongoing vaccination efforts for high-risk individuals.

The impact of the pneumococcal vaccination program (using the 23-valent pneumococcal polysaccharide vaccine, PPV23) in adults aged 65 and older, initiated in 2020, is also assessed. Results demonstrate a reduction in the incidence of invasive pneumococcal disease (IPD) due to vaccine serotypes, with vaccine effectiveness ranging from 48% to 73%. The program also showed a 19.7% impact on severe pneumococcal pneumonia, regardless of serotype. These findings support the continued use of pneumococcal vaccination to reduce severe disease in adults. The thesis also touches on the cost-effectiveness of different pneumococcal vaccines, noting the potential for newer vaccines like the 20-valent pneumococcal conjugate vaccine (PCV20) to be more cost-effective than PPV23.

Finally, the thesis discusses challenges in evaluating vaccination programs due to biases in observational data, emphasizing the need for robust study designs and improved data collection methods, such as a national adult vaccination registry. Utilizing multiple methodologies to estimate vaccine effectiveness strengthens the reliability of findings and informs evidence-based public health strategies.