Serological Response to the 2009 Pandemic Influenza A (H1N1) Virus for Disease Diagnosis and Estimating the Infection Rate in Thai Population

BACKGROUND: Individuals infected with the 2009 pandemic virus A(H1N1) developed serological response which can be measured by hemagglutination-inhibition (HI) and microneutralization (microNT) assays. METHODOLOGY/PRINCIPAL FINDINGS: MicroNT and HI assays for specific antibody to the 2009 pandemic virus were conducted in serum samples collected at the end of the first epidemic wave from various groups of Thai people: laboratory confirmed cases, blood donors and health care workers (HCW) in Bangkok and neighboring province, general population in the North and the South, as well as archival sera collected at pre- and post-vaccination from vaccinees who received influenza vaccine of the 2006 season. This study demonstrated that goose erythrocytes yielded comparable HI antibody titer as compared to turkey erythrocytes. In contrast to the standard protocol, our investigation found out the necessity to eliminate nonspecific inhibitor present in the test sera by receptor destroying enzyme (RDE) prior to performing microNT assay. The investigation in pre-pandemic serum samples showed that HI antibody was more specific to the 2009 pandemic virus than NT antibody. Based on data from pre-pandemic sera together with those from the laboratory confirmed cases, HI antibody titers ≥ 40 for adults and ≥ 20 for children could be used as the cut-off level to differentiate between the individuals with or without past infection by the 2009 pandemic virus. CONCLUSIONS/SIGNIFICANCE: Based on the cut-off criteria, the infection rates of 7 and 12.8% were estimated in blood donors and HCW, respectively after the first wave of the 2009 influenza pandemic. Among general population, the infection rate of 58.6% was found in children versus 3.1% in adults

Subjects and time of specimen collection.

<p>Subjects and time of specimen collection.</p

Comparison between HI antibody titers obtained from goose and turkey erythrocytes.

<p>Note: A/Thailand/104/2009(H1N1) was used as the test virus. GMT  =  geometric mean titer, CI  =  confidence intervals, RBC  =  red blood cells,</p><p>NA  =  Not applicable</p

Estimation on the infection rates of the 2009 pandemic influenza in different groups of subjects after the first epide mic wave.

<p>Infection rate is determined by HI titer ≥40 in adults or ≥20 in children.</p

Comparison between NT antibody titers obtained from RDE treated and RDE untreated sera.

<p>*There are significant differences between GMT of NT antibodies in RDE treated and untreated sera from all 7 serum settings (Wilcoxon Signed Ranks test, p<0.05).</p

Antibody titers by date after onset of symptom.

<p>(A) HI antibody titer; (B) NT antibody titer. Colored stacked bars give the proportion with titers of 10, 20, 40 and ≥80 while the line denotes the geometric mean titer with error bars indicating 95% confidence intervals.</p