Effect of Clinical and Virological Parameters on the Level of Neutralizing Antibody against Pandemic Influenza A Virus H1N1 2009

Background. Little is known about the antibody response in natural infection by the novel 2009 influenza A (H1N1) virus and its relationship with clinical and virological parameters. The relative lack of background neutralizing antibody against this novel virus provides a unique opportunity for understanding this issue. Methods. Case patients presenting with influenza-like illness who were positive for the pandemic H1 gene by reverse transcription polymerase chain reaction were identified. The serum antibody response was assayed by neutralizing antibody titer (NAT) against the virus in 881 convalescent donors. We retrospectively analyzed clinical parameters and viral load. Results. Ninety percent of the 881 convalescent donors had seroprotective titer of 1:40 or greater. The geometric mean titer of donors with convalescent NAT measured between day 21 and 42 was 1:101.1. Multivariate analysis by ordinal regression showed that pneumonia (odds ratio, 3.39; 95% confidence interval, 1.49–7.61; P = .004) and sputum production (odds ratio, 1.75; 95% CI, 1.01–3.01; P = .046) were the 2 independent factors associated with a higher level of convalescent NAT. Being afebrile on influenza presentation was associated with subsequent poor NAT (