Microneutralization reaction compared to hemagglutination inhibition assay to evaluate immunogenicity of influenza vaccines and influenza diagnostics

The aim of this study was to compare the results of the hemagglutination inhibition test (HI-test) and microneutralization reaction in detection of antibodies to influenza A(H1N1), A(H1N1)pdm09, and A(H3N2) viruses in human sera, as well optimize microneutralization reaction conditions. The proposed variant of microneutralization reaction is based on detecting decreased influenza virus reproduction in infected MDCK cells in the presence of virus-specific serum antibodies. Virus reproduction was evaluated by enzyme-linked immunosorbent assay in 96-well cell culture plates using type-specific anti-influenza A viruse NP-protein monoclonal antibodies. In parallel, in microneutralization reaction and HI-test paired sera collected from 205 volunteers inoculated with inactivated seasonal influenza vaccines were analyzed, as well as from 117 adult patients with laboratory-confirmed influenza. The rationale for treatment of human serum with receptor-destroying enzyme (RDE) was proved not only for HI-test, but also for microneutralization reaction. Compared to HI-assay, the microneutralization reaction displayed higher sensitivity. According to microneutralization data, seroconversion rates and increase in antibody titer against influenza A viruses in both vaccinated and infected persons were superior to HI-test data by 1.4–2.5-fold. Moreover, higher sensitivity of this method was of great importance for the diagnostics of disease caused by new pathogens. The efficacy of influenza A(H1N1)pdm09 serodiagnostics in PCRpositive patients was 1.5 times higher based on microneutralization reaction vs. HI-assay data. According to the data from early studies, it is commonly believed that 1/40 titer of flu-specific antibodies detected by HI-test is set as protective. However, a consensus on protective level for virus-specific antibodies in neutralization reaction has not been established yet. It was found that serum antibody levels detected by the proposed version of microneutralization reaction were significantly higher than those in HI-assay. In the post vaccination sera collected from vaccinated volunteers, average titers of virus neutralizing antibodies corresponding to 1/40 in HI-test were 1/195, 1/203, and 1/ 426–1/430 for influenza A(H1N1), A(H1N1)pdm09 and A(H3N2), respectively, whereas in influenza patients they were 1/285, 1/215 and 1/488, respectively. Thus, it was suggested to consider a threshold value for “conditionally protective” level of neutralizing antibodies in adult vaccinated volunteers or infected patients, an average titer 1/160 for A(H1N1) and A(H1N1)pdm09 viruses, as well as 1/320 — for A(H3N2) virus, which agree with data published elsewhere.