Currently commercialized seasonal influenza vaccines rely upon the supply of embryonated chicken eggs as the substrate for virus propagation. Seed virus strains bearing the recommended hemagglutinin and neuraminidase antigens are provided to vaccine manufacturers by one of the Control Agencies, such as the Center for Biologics Evaluation and Research (CBER) in the US, or the National Institute for Biological Standards and Control (NIBSC) in the UK. In order to maximize virus yield from eggs, reassortants of influenza A strains are often utilized. These possess the surface proteins from the desired wild-type H1N1 and H3N2 strains, while the other viral proteins are from a high-yielding strain such as A/Puerto Rico/8/34.