Experience with the clinical development of influenza vaccines for potential pandemics.

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 micro g 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 micro g haemagglutinin).

Immune response of human volunteers and animals to vaccination with egg-grown influenza A (H1N1) virus is influenced by three amino acid substitutions in the haemagglutinin molecule.

Inactivated subunit vaccines were prepared from high-growth reassortants derived from two separate egg isolates from a single clinical specimen of influenza A (H1N1) virus. One of these reassortants, NIB-14, was antigenically indistinguishable from isolates made in tissue culture, while the other, NIB-17, was antigenically different and typical of egg isolates. The viruses differed by three amino acid residues in the haemagglutinin (HA) molecule and the anti-HA serological response induced was studied in animal models and human volunteers. In the volunteer groups both vaccines induced very high levels of circulating haemagglutination inhibition antibodies but with different serological specificities. Both NIB-14 and NIB-17 vaccines induced high levels of cross-reactive antibodies capable of reacting with both strains, but only NIB-14 vaccine induced significant levels of strain-specific antibodies capable of reacting exclusively with the homologous strain. Antisera containing only cross-reactive antibodies proved as capable of virus neutralization as antisera containing high levels of strain-specific antibodies. We extended the argument that epidemic strains are antigenically more closely related to tissue culture isolates and established that viruses which differ by only single amino acids at critical points in the HA structure can induce a significantly different immune response when used as inactivated vaccines.