Letter to the Editor

New strategies to overcome the drawbacks of currently available flu vaccines

Vaccination represents the most efficient tool to control morbidity and mortality resulting from influenza infections in humans. The currently licensed influenza vaccines provide good protection levels in healthy adults, whereas lower protection is generally achieved in ageing individuals who are at a higher risk of developing severe clinical manifestations. Future improvements in influenza vaccines should address the needs of high risk groups including the elderly, small children and chronic patients. Recently, due to the increased incidence of avian influenza pandemic outbreaks, the prevention of a potential human influenza pandemic turned into another crucial issue in the influenza vaccination field. The development and validation of manufacturing processes for efficient and safe pandemic vaccines became one of the top priorities of health, regulatory and funding agencies all over the world. In the pandemic context, the development of novel vaccines administered via the mucosal route may play a significant role by reducing virus shedding from infected individuals. This chapter provides insights in the limitations of existing manufacturing processes, new approaches to overcome limitation in vaccine production, mechanisms of action of current vaccines and discuss potential strategies to improve the immunogenicity and efficacy of influenza vaccines.

Interfering vaccine (defective interfering influenza A virus) protects ferrets from influenza, and allows them to develop solid immunity to reinfection

Defective interfering (DI) virus RNAs result from major deletions in full-length viral RNAs that occur spontaneously during de novo RNA synthesis. These RNAs are packaged into virions that are by definition non-infectious, and are delivered to cells normally targeted by the virion. DI RNAs can only replicate with the aid of a coinfecting infectious helper virus, but the small size of DI RNA allows more copies of it to be made than of its full-length counterpart, so the cell produces defective virions in place of infectious progeny. In line with this scenario, the expected lethal disease in an influenza A virus-mouse model is made subclinical by administration of DI virus, but animals develop solid immunity to the infecting virus. Hence DI virus has been called an 'interfering vaccine'. Because interfering vaccine acts intracellularly and at a molecular level, it should be effective against all influenza A viruses regardless of subtype. Here we have used the ferret, widely acknowledged as the best model for human influenza. We show that an interfering vaccine with defective RNAs from an H3N8 virus almost completely abolished clinical disease caused by A/Sydney/5/97 (H3N2), with abrogation of fever and significant reductions in clinical signs of illness. Animals recovered fully and were solidly immune to reinfection, in line with the view that treatment converts the otherwise virulent disease into a subclinical and immunizing infection.

Interfering vaccine: a novel antiviral that converts a potentially virulent infection into one that is subclinical and immunizing

Infection with human influenza A virus can reach cataclysmic levels, with 40 or more million deaths arising from the 1918 pandemic. Preventative and therapeutic measures have improved since that time, but new approaches are needed. Here, we describe one such new approach--the interfering vaccine, which has two activities--it prophylactically prevents influenza, and at the same time converts an otherwise lethal infection into one that is avirulent and immunizing. Mice treated in this way develop a solid immunity that protects them against a subsequent challenge with homologous virus, and to a lesser extent from challenge with heterologous influenza A viruses. The interfering vaccine comprises non-infectious, defective interfering (DI) influenza A virus. Prophylaxis is mediated directly by DI RNA, and results from interference with the replication of the infecting virus. However, interference is incomplete, and there is sufficient wild-type virus multiplication to stimulate a virus-specific natural immunity. As the replication mechanism is common to all influenza A viruses, an interfering vaccine should protect from, and permit immunity to be developed to all influenza A viruses. Indeed, we demonstrate protection against two viruses with antigenically unrelated HA and NA proteins. Thus an interfering vaccine, unlike the conventional vaccine, is independent of the antigenicity of the infecting virus. In principle, interfering vaccines derived from other virus systems could also be developed.