The relevance for Public Health of the mid-term and interim analyses of seasonal influenza vaccination effectiveness

Worldwide, influenza is commonly recognized as a respiratory virus that causes a high number of deaths every year, especially among elderly adults and people with chronic illnesses. In order to reduce morbidity and mortality, many countries have implemented, in the last decades, universal influenza vaccination programs with special attention on groups at increased risk of developing the disease, such as pregnant women, children aged 6–59 months, elderly adults and individuals with specific chronic medical conditions. In real-world conditions, observational (nonrandomized) studies on vaccine effectiveness (VE) could measure,the risk reduction of contracting a disease and are considered the gold standard to estimate the VE of seasonal influenza vaccines