The serological response of experimental animals to inactivated whole and split influenza virus vaccines

Use of single radial haemolysis for assessing antibody response to influenza virus vaccines in animals

The value of the single radial haemolysis (SRH) test as a possible replacement for the haemagglutination-inhibition (HAI) test for the estimation of antibodies against influenza was assessed in three animal models. The serum antibody response was measured by both assay systems; correlation of the two tests was assessed using regression analysis. The study showed that when the response to a single immunisation was determined, the ferret model gave satisfactory correlation of SRH and HAI, whilst in the mouse and hamster models poor correlation was observed. Correlation was only improved in the mouse model when an immunisation schedule that mimicked the human situation of a background exposure to different strains of influenza virus was used. Since influenza vaccine efficacy is usually assessed in animals using a single immunisation we suggest that the SRH is not acceptable for use in either hamsters or mice, but is acceptable where the ferret model is involved.

Interference following dual inoculation with influenza A (H3N2) and (H1N1) viruses in ferrets and volunteers

The effects of simultaneous inoculation with two attenuated influenza A viruses was studied in ferrets and volunteers. Groups of ferrets were inoculated with an influenza A (H3N2) or (H1N1), virus or a combination of both viruses: the temperature response, serum and local antibody response, and the change in nasal wash protein concentration was determined. The results showed that both viruses were attenuated for ferrets, and that inoculation with both viruses together did not cause clinical reactions. Serological studies on paired serum samples obtained from ferrets showed that both viruses when given separately infected all the inoculated animals; however, dual infection resulted in all ferrets being infected with the influenza A (H3N2) virus strain, but this infection interfered with infection by the influenza A (H1N1) strain. Similar investigations were carried out in volunteers. Again, the clinical reactions and temperature response of volunteers to infection by one or other of the viruses showed both strains to be attenuated for man even when given together. In addition, no adverse clinical reactions were seen in volunteers inoculated with both viruses simultaneously. Serum antibody studies showed that infection by influenza A (H1N1) virus interfered with infection by the influenza A (H2N2) virus strain. These results show evidence of interference by influenza A viruses; however, the direction of interference was one-way, and differed for ferrets and for volunteers.

Infant rat model of attenuation for recombinant influenza viruses prepared from cold-adapted attenuated A/Ann Arbor/6/60

The pathogenicity of 6 wild-type influenza A viruses and 21 recombinant strains prepared from wild-type viruses and cold-adapted A/Ann Arbor/6/60 virus for infant rats was determined. Thus, the titers of virus present in the turbinates and lungs of virus-infected animals was measured serially for 5 days after intranasal infection, and the ability of virus strains to promote subsequent systemic bacterial infection by Haemophilus influenzae was measured at 48 h after virus infection. The results obtained were assessed with reference to the genetic constitution of the virus strains and to virus virulence for volunteers. The results showed that virulent viruses grew to relatively high titers in rat turbinates and significantly promoted systemic infection by H. influenzae. In contrast, attenuated strains grew to lower titers and failed to promote systemic H. influenzae infection. For the strains tested, the results showed clear differences for attenuated and virulent strains, and the model was a reliable indication of virulence for humans. Although the virulent strains tended to grow to higher titers in rat lungs than did attenuated strains, exceptions were found, and this measurement could not reliably discriminate virulent and attenuated virus strains. The results suggest that infant rats can be used to assess the virulence of cold-adapted recombinant influenza virus strains, and thus, they can facilitate the development of such strains for vaccine production.

Antigenicity in hamsters of inactivated vaccines prepared from recombinant influenza viruses

Inactivated vaccines prepared form influenza virus strains obtained by the recombination of A/PR/8/34 (H1N1) or A/FM/1/47 (H1N1) viruses with A/Victoria/3/75 (H3N2) virus, were tested for their antigenicity in hamsters. The parental origin of the genes of each cloned recombinant virus was determined by polyacrylamide gel electrophoresis, and vaccines prepared from each strain by concentration, purification on sucrose density gradients and inactivation with formalin. All the recombinant strains used in these studies possessed surface haemagglutinin and neuraminidase antigens derived from the A/Victoria/75 parent strain. On inoculation into hamsters, at equivalent concentrations, these vaccines varied in their ability to induce haemagglutination-inhibiting (HI) antibodies in the serum. This variation was not dependent on concentration and was observed using neutralization and single radial haemolysis, as well as HI. The possible reasons for the findings are discussed.

The amantadine-sensitivity of recombinant and parental influenza virus strains

Several wild-type influenza A strains together with recombinants derived from these strains, were tested for sensitivity to amantadine using the in vitro techniques of inhibition in egg-bit culture and plaque reduction in MDCK cells. The results obtained were analysed with reference to the derivation of the recombinants. Susceptibility to amantadine was related to the gene coding for matrix protein, and these data are in agreement with previous reports of studies using other series of influenza viruses.

Comparative immunogenicity of live influenza viruses and their solubilized neuraminidases: results of mouse protection experiments

Antigenicity and immunogenicity of three influenza virus strains A/PR/8/34 (H1N1). A/Hong Kong/1/68 (H3N2) and A/Port Chalmers/1/73 (H-3H2) were assayed comparatively with their corresponding neuraminidase isolated by proteolysis, and with the recombinant virus X-42 (Heq1 N2). It was concluded that intranasal immunization of mice with live virus induced heterologous immunity. Except in homologous neuraminidase-vaccinated mice, the subunit always was shown less effective and demonstrated a significantly lower antibody response than the corresponding whole virus.