Influenza vaccines: an introduction

Seasonal influenza is a leading cause of severe respiratory infections and deaths. Type A influenza can cause severe respiratory disease, death and pandemics. Influenza B accounts for about a quarter of the annual number of seasonal flu cases, but does not cause pandemics. Influenza C viruses cause mild symptoms in humans. Evidence suggests Influenza D does not cause disease in humans. During an influenza infection, the immune system produces inflammatory mediators, which are largely responsible for the systemic symptoms and complications. Vaccines remain the most effective way to prevent influenza-related respiratory disease and non-respiratory complications despite antivirals. Numerous studies support the benefits of influenza vaccination, especially in high-risk groups. However, the effectiveness of the influenza vaccine varies markedly depending on the match between the circulating strains and those in the vaccine, and the recipient's characteristics. Influenza vaccines are, in general, well-tolerated. Research could lead to more effective and, potentially, universal influenza vaccines.