Strengthening influenza surveillance systems
Authors:   , El Guerche-Seblain Clotilde 
Published: March 2023

Surveillance systems appear to be a crucial tool to monitor the evolution of epidemics. The thesis provides recommendations for strengthening influenza surveillance systems based on findings from five real-world evidence studies: an assessment of the quality of country influenza surveillance systems, a prospective surveillance study in a large hospital, a retrospective analysis of national hospital discharge databases, and a descriptive analysis of influenza in children, included in the placebo arm of a vaccine efficacy randomized controlled trial.
In chapter 2, three national influenza surveillance systems in Asia were compared and their adherence to World Health Organization (WHO) guidance. Experts issued recommendations for their improvement. Recommendations encompasses introducing new surveillance tools, broadening the use of specific existing tools, collecting and sharing data on virus characteristics, developing immunization status registries, improving public health communication.
In chapter 3, we examined influenza virus circulation in countries in the Asia-Pacific region and compared with the timing of influenza vaccination. Data were obtained from the WHO FluNet database for 2010–2017 from 19 countries. Peak circulation coincided with the winter seasons in most countries, although patterns were less clear in the inter-tropical area. The WHO-recommended timing for influenza vaccination was appropriate in 12 countries, inappropriate in 3 countries and inconclusive for 4 countries due to unclear seasonality of influenza virus circulation.
In chapter 4, we analyzed local surveillance data based on a prospective surveillance study at the Hospital Edouard Herriot in Lyon, France over 11 years, and estimated the incidence and trends of hospital-acquired laboratory confirmed influenza infections among adult. An advanced age and an underlying disease were important characteristics of HAI. HAI occurred at a rate of 1 in 13 with influenza-like-illness in hospital. The HAI incidence was the highest in the geriatric ward and increased over time. HAI rates approximated hospital onset Clostridium difficile reported rates.
In chapter 5, we examined the occurrence and burden of influenza in healthy children using data from a phase III placebo-controlled influenza vaccine trial. Data from 2,210 participants were analysed. The overall influenza attack rate was 11.5%. The most common influenza virus was A(H3N2) (40.7%), followed by B/Yamagata (23.6%), A(H1N1) (18.6%), and B/Victoria (8.0%). Grade 3 fever was reported in 24.3% of confirmed episodes, acute lower respiratory infection in 8.7%, acute otitis media in 6.1%, and pneumonia in 1.9%. Antibiotics were prescribed for 41.4% of influenza episodes. More than half of the episodes (57.0%) resulted in outpatient visits and for 1.1% of episodes in hospitalization.
In chapter 6, we characterized the cumulative risk of comorbidities and age on severe outcomes in Mexico and Brazil. We used national hospital discharge data from Brazil (SIH/SUS) from 2010-2018 and Mexico (SAEH) from 2010-2017 to estimate the number of influenza admissions using ICD-10 discharge codes. In Brazil, the median duration of hospital stay was 3.0 days, ICU admission rate was 3.3%, and in-hospital Case fatality rate was 4.6% (95% CI, 4.5-4.7). In Mexico, the median duration of stay was 5.0 days, ICU admission rate was 1.8%, and in-hospital CFR was 6.9%. We found that age and comorbidities, especially cardiovascular disease, are cumulatively associated with more severe outcomes.
In chapter 7 the main findings are discussed. National or regional surveillance data should be used to inform who and when to vaccinate and which vaccines to use in order to consider epidemiological and demographics countries’ variability. Priority target groups for influenza vaccination should be defined based on evidence about burden of disease and characteristics of the circulating virus. While we are still fighting the Covid-19 pandemic, our knowledge of influenza surveillance systems and their capacities should integrate surveillance of the main respiratory viral infections.