John M. Wood

The production of seasonal and pandemic influenza vaccines depends on the timely availability of suitable reference viruses. Seasonal vaccines are traditionally produced from high‐growth reassortant viruses, which have been derived empirically using well‐established techniques. However, it is not possible to use such approaches in deriving vaccine reference viruses from highly pathogenic H5N1 viruses and alternative techniques such as reverse genetics must be employed.

Over the past eight years, cases of human infection with highly pathogenic avian influenza viruses have raised international concern that we could be on the brink of a global influenza pandemic. Many of these human infections have proved fatal and if the viruses had been able to transmit efficiently from person to person, the effects would have been devastating. How can we arm ourselves against this pandemic threat when these viruses are too dangerous to use in conventional vaccine production? Recent technological developments (reverse genetics) have allowed us to manipulate the influenza virus genome so that we can construct safe, high-yielding vaccine strains. However, the transition of reverse-genetic technologies from the research laboratory to the manufacturing environment has presented new challenges for vaccine manufacturers as well as veterinary and public health authorities.

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.

During normal interpandemic influenza seasons, immune responses to vaccines are quite predictable and meet the licensing criteria of the European Union (EU) Committee for Proprietary Medicinal Products (CPMP). In a pandemic situation, large sections, if not all of the community will be immunologically naïve and therefore new immunisation strategies will be needed. In 1976 and 1977 H1N1 vaccines were prepared and tested clinically. To stimulate 'protective' antibody responses, two doses of vaccine were needed in people below the age of 24 years (no previous experience of H1N1 virus), whereas one conventional dose was adequate in older people. In 1997, the highly pathogenic avian influenza H5N1 virus caused widespread concern when it infected man, with lethal effects. Due to safety concerns it was necessary to adopt new strategies for vaccine development and one such strategy was to produce vaccine from an avirulent H5N3 virus, A/Duck/Singapore-Q/F119-2/97. Clinical trials of a subunit vaccine prepared from A/Duck/Sing/97 virus revealed that even two doses of twice the normal vaccine concentration (i.e. 30 µg haemagglutinin) were poorly immunogenic, whereas an H5N3 vaccine adjuvanted with microfluidised emulsion (MF) 59 stimulated antibody levels that complied with CPMP criteria after two half strength doses (i.e. 7.5 µg haemagglutinin).

Pandemic influenza presents special problems for vaccine development. There must be a balance between rapid availability of vaccine and the safeguards to ensure safety, quality and efficacy of vaccine. Vaccine was developed for the pandemics of 1957, 1968, 1977 and for the pandemic alert of 1976. This experience is compared with that gained in developing vaccines for a possible H5N1 pandemic in 1997–1998. Our ability to mass produce influenza vaccines against a pandemic threat was well illustrated by the production of over 150 million doses of ‘swine flu’ vaccine in the USA within a 3 month period in 1976. However, there is cause for concern that the lead time to begin vaccine production is likely to be about 7–8 months. Attempts to reduce this time should receive urgent attention. Immunogenicity of vaccines in pandemic situations is compared over the period 1968–1998. A consistent feature of the vaccine trials is the demonstration that one conventional 15μg haemagglutinin dose of vaccine is not sufficiently immunogenic in naive individuals. Much larger doses or two lower doses are needed to induce satisfactory immunity. There is some evidence that whole–virus vaccines are more immunogenic than split or subunit vaccines, but this needs substantiating by further studies. H5 vaccines appeared to be particularly poor immunogens and there is evidence that an adjuvant may be needed. Prospects for improving the development of pandemic vaccines are discussed.