Intro: Enterovirus A71 (EV-A71), the major pathogen causing hand, foot, and mouth disease, affects young children in the Asia-Pacific region with cyclical epidemics. Numerous studies estimated seroprevalence of EV-A71 neutralizing antibodies. However, evidence of incidence rates from longitudinal studies is scarce. This study aims to examine age-specific incidence rates of EV-A71 infection, and to investigate dynamics of serum neutralizing antibody response to infection by following a birth cohort. Methods: We recruited 759 neonates in northern Taiwan from June 2006 to December 2009 and followed these children for 7 years. Study participants’ sera were collected at birth, 6, 12, 24, 36, 48, 60, 72 and 84 months of age (m) for conducting neutralization test on EV-A71 to measure age-specific incidence rates and to observe reinfections at different ages. Findings: A total of 576 children returning for follow-up were analyzed from 2006 to 2013. The incidence rates increased gradually from 0.68 per 100 personyears at 6m to 19.33 at 60m, and reduced to 9.57 at 72m. The cumulative incidence rate of primary and secondary infections by 84m reached 37.97% and 6.96%, respectively. Of the 123 children with a primary infection, 43% were asymptomatic. The cumulative negative conversion rate of serum neutralizing antibody was 12%, 16.84%, and 23.51% in the first, second, and third year post primary infection, respectively. The decay rate of neutralizing antibody titer was estimated as 4.37% per month, and the half-life of infection-induced neutralizing antibody was estimated as 15.9 months. Discussion: Our findings suggest that cumulative incidence of primary and secondary infections by 7 years of age was about 38% and 7% after experiencing 2 epidemics. We also characterized the dynamics of neutralizing antibody titers after EV-A71 infections. Conclusion: These findings provide critical information for designing efficacy trials of EV-A71 vaccines and formulating vaccination policy.

Intro: Enterovirus-A71 (EV-A71), a major cause of hand, foot, and mouth diseases (HFMD), has caused several outbreaks in Vietnam after its first identification in 2003. In 2011, a large-scale EV-A71 outbreak occurred in Southern Vietnam. To characterize epidemiological patterns of EV-A71 in Southern Vietnam, we conducted a hospital-based enterovirus surveillance in Children's Hospital No. 1 (CH1) Ho Chi Minh City over a 9-year period from 2012- 2020. Methods: Clinical samples (throat swabs and serum) were collected from CH1 inpatients with suspected enteroviruses infection for virus isolation in CH1. VP1- CODEHOP, a molecular assay, was performed for detection and genotyping of enteroviruses in National Health Research Institutes. Full genomes of selected EV-A71 viruses were also sequenced for phylogenetic analysis using Neighbor- joining method in MEGA 11 software. Findings: From 2012 to 2020, a total of 3184 HFMD inpatients were tested and 1693 of them (53%) were positive for enterovirus using VP1-CODEHOP, including 368 (11.56%) EV-A71 cases. Based on virus isolation conducted in 2012-2019, EV-A71 detection rate was 6.79% (177/2607). During the study period, EV-A71 was the top serotype detected in 2013, and 2016∼2018. Overall, the other prevalent serotypes were CV-A2, CV-A6, CV-A10 and CV-A16. Partial VP1 genes of 368 EV-A71 isolates were sequenced, 86.4% were genotype B5 and 13.6% C4. The predominant genotype was C4 in 2012 and shifted to B5 afterward. Based on full genome sequencing of five EV-A71 strains (3 B5 and 2 C4), the genotype C4 viruses were phylogenetically related to the C4 viruses isolated in China, genotype B5 were phylogenetically related to the B5 virus circulated in Malaysia, Singapore, and Taiwan. Conclusion: EV-A71 outbreaks occurred every 2-3 years in Southern Vietnam and the predominant genotype circulating in Southern Vietnam was genotype B5. Development of EV-A71 vaccine is the priority and other prevalent serotypes should be included for developing multivalent vaccines in the future.

Background: Introduction: Large-scale epidemics of enterovirus A71 (EV-A71) genotype occurred with different genotypes in 2005 (genotype C5) and 2011 (genotype C4a) in southern Vietnam. B5 had replaced C4 as the predominant genotype since 2013; however, some evidence suggested the reemergence of C4 since 2018. This study aims to investigate genetic and antigenic evolution of EV-A71 in south Vietnam. Methods and materials: Methods: National Health Research Institutes (NHRI), Taiwan has collaborated with Children's Hospital 1 (CH1), Ho Chi Minh City (HCMC), Vietnam to establish a hospital-based surveillance system of EV-A71 infections since 2012. Residual sera and throat swabs were collected from hand-foot-mouth disease (HFMD) inpatients for virological tests including virus isolation tested in CH1 using RD cells, and semi-nested RT-PCR (CODEHOP) and serum neutralization test performed in NHRI. Results: Based on virus isolation, EV71 positive rates were 6.8%, 16.0%, 2.7%, 9.6%, 5.2%, 1.6%, and 3.5% from 2012 to 2018, respectively, which are significantly lower than the positive rates tested using CODEHOP in the corresponding years (14.7%, 20.1%, 6.7%, 11.0%, 22.8%, 19.6%, and 6.0%). The results of genetic analysis indicated EV-A71 was the predominant serotypes in 2013, 2016 and 2017 and EV-A71 genotypes shifted from C4 in 2012 to B5 in 2013–2017. We found C4a and B5 had comparable number of virus isolates in 2018. Based on phylogenetic analysis, the genotype C4a viruses were likely imported from China and the genotype B5 viruses could be from southern Asia. Interestingly, the genotype B5 and C4a viruses circulating in HCMC have similar antigenicity based on neutralization tests using sera collected from children infected with EV-A71. Conclusion: In conclusion, international spreading of EV-A71 is common in southern Vietnam and EV-A71 epidemics cyclically occurred in 2–3 years since 2011. Enterovirus surveillance and vaccine development is urgently needed in Vietnam.

Background: Human enteroviruses are classified into four species (A, B, C and D) and include over 100 serotypes. Since 1997, Enterovirus 71(EV71) usually causes self-limited infections with non-specific symptoms and manifests hand-food-month disease (HFMD) or herpangina in children. EV71 have caused severe life-threatening outbreaks in young children in Asian. EV71 circulation among Vietnamese children was first documented in 2005. Methods & Materials: In 2011, there is a big outbreak of EV71 in Vietnam with more than 5,000 inpatients in Children Hospital No.1 (CH1), and 32 fetal cases. National Health Research Institutes cooperates with CH1 to establish hospital-based surveillance of enterovirus in HCM City in 2011. Children <10 years of age who develop HFMD and were admitted to HCM CH1 were collected throat swabs and sera. Throat swabs were used for virus isolation and Sera were used to measure neutralizing antibody against EV71 in Taiwan NHRI. Results: Enterovirus isolate rate with HFMD-related inpatients including 38.9% (21/54) in 2011, 19.3% (79/409) in 2012, 42.0% (173/412) in 2013 and 12% (12/100) on June, 2014. Among them, EV71 positive rate from 2011 to June, 2014 were 29.6%, 6.8%, 16.0% and 5%, respectively. The age-specific seropositive rates increase from 15.2% at <0.5 years of age to 17.2, 24.0, 29.4, 58.6, 62.3, 66.1, 77.6% at 0.5-0.9, 1-1.9, 2-2.9, 3-3.9, 4-4.9, 5-5.9 and 6-6.9 years of age, respectively. Conclusion: The predominant genotype shifted from C4 in 2011 to B5 in 2013. Risk of EV71 infections in Vietnam increased after 6 months of age. Vietnamese children in HCM City acquired EV71 infections at early age and vaccine development in Vietnam should target young children.

Objective: Enterovirus 71 (EV71) is causing life-threatening outbreaks in tropical Asia. In Taiwan and other tropical Asian countries, although nationwide EV71 epidemics occur cyclically, age-specific incidence rates of EV71 infections that are critical to estimate disease burden and design vaccine trials are not clear. A nationwide EV71 epidemic occurred in 2008-09 in Taiwan, which provided a unique opportunity to estimate age-specific incidence rates of EV71 infections.Study Design: We prospectively recruited 749 healthy neonates and conducted follow-ups from June 2006 to December 2009. Sera were obtained from participants at 0, 6, 12, 24, and 36 months of age for measuring EV71 neutralizing antibody titers. If the participants developed suspected enterovirus illnesses, throat swabs were collected for virus isolation.Results: We detected 28 EV71 infections including 20 symptomatic and 8 asymptomatic infections. Age-specific incidence rates of EV71 infection increased from 1.71 per 100 person-years at 0-6 months of age to 4.09, 5.74, and 4.97 per 100 person-years at 7-12, 13-24, and 25-36 months of age, respectively. Cumulative incidence rate was 15.15 per 100 persons by 36 months of age, respectively.Conclusions: Risk of EV71 infections in Taiwan increased after 6 months of age during EV71 epidemics. The cumulative incidence rate was 15% by 36 months of age, and 29% of EV71 infections were asymptomatic in young children.

Background: Enterovirus 71 (EV71) is causing life-threatening hand-foot-mouth disease (HFMD) involving neurological and cardiopulmonary complications in Asian children. Phylogenetically, EV71 viruses can be classified into 3 genogroups (A, B and C) and 11 genotypes (A, B1∼B5 and C1∼C5). In Taiwan, nationwide EV71 epidemics with different predominant genotypes occurred in 1998 (C2), 2000–2001 (B4), 2004–2005 (C4) and 2008 (B5) but the mechanism is not clear. In early 2007, genotype C5 viruses were isolated sporadically and EV71 epidemic did not begin until genotype B5 viruses were detected in late 2007. This study was conducted to measure cross-reactive neutralizing antibody response to EV 71 Infection in Taiwanese young children to explore the mechanism of the genotype replacement in Taiwan, which is critical to the selection of vaccine strains. Methods: We prospectively conducted cohort study to follow up healthy neonates starting from June 2006 and sera were collected from participating children for measuring EV71 neutralizing antibody titers at birth and at 6, 12, 24, and 36 months of age. Throat swabs were collected from participating children developing herpangina or HFMD for virus isolation. Isolated EV71 viruses were genotyped using VP1 gene sequences. Results: In 2008–09, 24 children developed EV71 neutralizing antibody seroconversion, including 11 symptomatic and 13 asymptomatic infections. Five EV71 viruses were isolated from the symptomatic cases and all belongs to genotype B5. These B5 viruses are phylogetically related to B5 viruses circulating in the South-Eastern Asia recently. Thirtyone post-infection sera collected from the 24 seroconverted children were measured neutralizing antibody titers against genotype A, B4, B5, C2, and C4 viruses. Geometric mean (95% confidence intervals) of neutralizing antibody titers against these viruses were 52 (38–72), 150 (109–205), 234 (176–311), 114 (82–158), and 105 (76–144), respectively. Serological tests show that children infected with B5 viruses have lower neutralizing antibody titers against A, C2 and C4 viruses than B5 virus (p < 0.05, t-test). Conclusion: Antigenic differences could be detected between enterovirus 71 viruses in different genogroups but not in the same genogroup using children post-infection sera. Significant antigenic differences between B5 and C4 viruses may explain the genogroup replacement occurring in the 2008 epidemic. Abstracts for SupplementInternational Journal of Infectious DiseasesVol. 14Preview Full-Text PDF Open Archive

Background: Clinical spectrum of EV71 infection ranges from mild hand-foot-mouth disease (HFMD) to severe cases with central nervous system and cardiopulmonary involvements. In 1998, Taiwan suffered a nation-wide EV71 epidemic with 405 reported severe cases and 78 fatal cases. Since then, EV71 has been endemic in Taiwan and development of EV71 vaccines has become a national priority. This cohort study was designed to understand decline of maternal EV71 neutralizing antibody titers and age-specific incidence rates of EV71 infection in Taiwan, which will be crucial for designing efficacy trials of EV71 vaccines. Methods: This study is a prospective longitudinal cohort study and was approved by the Institutional Review Boards in CGMH and NHRI. Pregnant women having prenatal exams were invited to participate. Sera were collected from participating mother/baby for measuring EV71 neutralizing antibody titers in the following schedules: last trimester from mother, umbilical cord blood at birth, and children at 6, 12, 24, 36, and 48 months old. Results: The recruitment started in June 2006. By January 2008, 201 longitudinal serum specimens collected from mothers, neonates, and 6-month-old babies (one mother delivered twins) have been measured with EV71 neutralizing antibody titers. The seropositive rates (percentage of antibody titer ≥ 1:8) in mothers, neonates, and 6-month-old babies were 61%(122/201), 46%(93/202), and 0%(0/202), respectively. Geometric mean titers (95% confidence intervals) of serum EV71 antibody in mothers and neonates were 10.2 (8.7, 11.5) and 8.5 (7.4, 9.7), respectively. Conclusions: In northern Taiwan, only 46% of neonates have protective antibody against EV71 and these maternal antibody titers have declined to undetectable level (<8) by 6 months of age. Historical data in the 1998 epidemic also showed that infants under 1 year old had the highest risk of severe EV71 infection. Therefore, development of EV71 vaccines in Taiwan should target infants under 6 months old.