BackgroundIndividuals 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 FindingsMicroNT 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/SignificanceBased 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.

Background: Enterovirus A71 (EV-A71) and coxsackievirus A16 (CV-A16) are the major causative agents of hand, foot and mouth disease (HFMD) worldwide, particularly in the Asia-Pacific region. Several strains have emerged, circulated, and faded out over time in recent decades. This study investigated the EV-A71 and CV-A16 circulating strains and replacement of genotypes/subgenotypes in Thailand during the years 2000-2017. Methods: The complete VP1 regions of 92 enteroviruses obtained from 90 HFMD patients, one asymptomatic adult contact case, and one encephalitic case were sequenced and investigated for serotypes, genotypes, and subgenotypes using a phylogenetic analysis. Results: The 92 enterovirus isolates were identified as 67 (72.8%) EV-A71 strains comprising subgenotypes B4, B5, C1, C2, C4a, C4b and C5, and 25 (27.2%) CV-A16 strains comprising subgenotypes B1a and B1b. Genotypic/subgenotypic replacements were evidenced during the study period. EV-A71 B5 and C4a have been the major circulating strains in Thailand for more than a decade, and CV-A16 B1a has been circulating for almost two decades. Conclusions: This study provides chronological data on the molecular epidemiology of EV-A71 and CV-A16 subgenotypes in Thailand. Subgenotypic replacement frequently occurred with EV-A71, but not CV-A16. Monitoring for viral genetic and subgenotypic changes is important for molecular diagnosis, vaccine selection, and vaccine development.

This study monitored the long-term immune response to severe acute respiratory syndrome coronavirus (SARS-CoV)-2 infection in patients who had recovered from coronavirus disease (COVID)-19. Anti-nucleocapsid immunoglobulin G (anti-N IgG) titer in serum samples collected at a single (N = 302) or multiple time points (N = 229) 3–12 months after COVID-19 symptom onset or SARS-CoV-2 detection in respiratory specimens was measured by semiquantitative chemiluminescent microparticle immunoassay. The 531 patients (966 specimens) were classified according to the presence or absence of pneumonia symptoms. Anti N IgG was detected in 87.5% of patients (328/375) at 3 months, 38.6% (93/241) at 6 months, 23.7% (49/207) at 9 months, and 26.6% (38/143) at 12 months. The anti-N IgG seropositivity rate was significantly lower at 6, 9, and 12 months than at 3 months (P < 0.01) and was higher in the pneumonia group than in the non-pneumonia/asymptomatic group at 6 months (P < 0.01), 9 months (P = 0.04), and 12 months (P = 0.04). The rate started to decline 6–12 months after symptom onset. Anti-N IgG sample/cutoff index was positively correlated with age (r = 0.192, P < 0.01) but negatively correlated with interval between symptom onset and blood sampling (r =  − 0.567, P < 0.01). These findings can guide vaccine strategies in recovered COVID-19 patients.