According to the World Health Organization (WHO), seasonal influenza epidemics affect an estimated 5–15% of the total population worldwide, with 3–5 million cases of severe illness, resulting in 250,000–500,000 deaths. However, few data are available for many parts of the world where active surveillance is lacking. In addition, the viruses and the severity of influenza epidemics vary greatly between years and geographical areas. To address the rapidly evolving antigenicity of circulating influenza viruses, twice annually, the WHO re-evaluates the viruses that should be included in the seasonal influenza vaccines. To inform policy decisions, national health authorities need to understand the burden of influenza disease and the impact of current vaccination programs in their countries.
High-quality, active surveillance networks are needed to better understand influenza epidemiology and therefore better control influenza epidemics. Data from existing sentinel physician networks are used in several countries to conduct annual studies on the effectiveness of vaccines in preventing medically attended influenza-like illness (ILI). These networks, however, do not collect data on the impact of influenza infection on hospitalization or on the impact of influenza vaccines on influenza-related hospitalization, which substantially influence evaluation of the benefits and cost-effectiveness of influenza vaccines.
Active surveillance networks are also powerful advocacy instruments for highlighting the often-underestimated impact of influenza. While hospital surveillance systems already exist for detecting outbreaks of respiratory viruses, few focus on the actual burden of serious influenza cases using the specific outcome of laboratory-confirmed influenza; instead, the burden is most often estimated from hospital databases using criteria prone to various biases.

The Global Influenza Hospital Surveillance Network (GIHSN) was initiated in 2011 to fill this gap in epidemiology and public health knowledge. The GIHSN is a public-private partnership between Sanofi Pasteur, FISABIO-Salud Pública, and several coordinating sites affiliated with national health authorities . In accordance with WHO recommendations, coordinating sites are selected based on their motivation, geographic representativeness, ability to conduct epidemiological studies, availability of laboratory facilities, and experience in influenza surveillance. Each coordinating site supervises a group of one to seven hospitals in its country or geographical region and follow a core reference protocol. The GIHSN has three main objectives: (i) evaluate the burden of severe influenza disease, defined as hospitalization related to community-acquired influenza or complications following an influenza infection; (ii) quantify the distribution of the different influenza viruses (A(H1N1), A(H3N2), B/Yamagata, and B/Victoria) among these severe cases; and (iii) measure the effectiveness of influenza seasonal vaccines to prevent these hospitalizations using a test-negative design.