Influenza vaccines are licensed in the European Union (EU) by means of the mutual recognition procedure, which includes a "fast track" variation for rapid approval of new vaccine strains. This "fast track" procedure involves an annual clinical trial and assessment of immunogenicity according to serological criteria published by the EU Committee for Proprietary Medicinal Products (CPMP). A survey of vaccine clinical trials in the elderly over the past four years has demonstrated that inter-laboratory differences in serology results can affect compliance with CPMP licensing criteria. Thus even well established immune correlates of influenza vaccine efficacy can be problematic. For the next generation of vaccines, however, where mucosal or cell-mediated immunity may contribute towards vaccine efficacy, the serological correlates may be inadequate.