Health benefits, risks, and cost-effectiveness of influenza vaccination in children.

Annual vaccination of all children 6-59 months of age is recommended in the United States but not in most of Europe. This paper reviews issues surrounding the epidemiology of influenza and vaccine effectiveness relevant to the determination of vaccination policy. Most clinical trials of vaccines (and treatments) that provide the evidence for current policy took place in the 1990s when rates of influenza-like illness (ILI) were twice those reported in recent years. The impact of influenza in the community is concealed by the variety of diagnoses appropriate to describing acute respiratory infections. Even in influenza virus active periods diagnoses of coryza, acute bronchitis, and otitis media made by general practitioners greatly exceed those of ILI. Respiratory syncytial virus presents particular problems because it often circulates at the same time as influenza. Thus, the diagnostic uncertainty and variety of respiratory pathogens causing similar illnesses are major confounders when estimating influenza vaccine effectiveness. Although meta-analyses have cast doubt on the wisdom of the universal vaccination of children, high-quality clinical trials have demonstrated efficacy against laboratory-confirmed infection. The distinction between this positive benefit and the wider issue of effectiveness against ILI in the community poses difficulties for determining policy. Mathematical models examining this issue are populated with data that are mostly estimated: therefore, the sensitivity analysis is critical and the conclusions are invariably accompanied by reservations. National policies based on demonstrated cost effectiveness are desirable, but these should not become a barrier to parents wishing to secure a benefit for their child or family, provided there is clear evidence of clinical efficacy.