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Background: The Plasmodium vivax Reticulocyte Binding Protein (PvRBP) family is involved in red blood cell recognition and members of this family are potential targets for antibodies that may block P. vivax invasion. To date, the acquisition of immunity against PvRBPs in low malaria transmission settings and in a broad age group of exposed individuals has not been investigated. Methodology/Principal findings: Total IgG antibody levels to six members of the PvRBP family (PvRBP1a, PvRBP1b, PvRBP2a, PvRBP2b, a non-binding fragment of PvRBP2c (PvRBP2cNB) and PvRBP2-P2) were measured in samples collected from individuals living in two regions of low P. vivax endemicity in Brazil and Thailand. In both settings, levels of total IgG to PvRBP1a, PvRBP2b, PvRBP2cNB, and PvRBP2P-2 increased significantly with age (rho = 0.17–0.49; P<0.001). IgG responses to PvRBP1a, PvRBP2b and PvRBP2cNB were significantly higher in infected individuals by using Wilcoxon’s signed-rank test (P<0.001). Of the six PvRBPs examined, only antibodies to PvRBP2b were associated with protection against clinical malaria in both settings. Conclusion/Significance: Our results indicate that PvRBP2b warrants further preclinical development as a blood-stage vaccine candidate against P. vivax. Total IgG responses to PvRBPs were also shown to be promising immunological markers of exposure to P. vivax infection. [ABSTRACT FROM AUTHOR]

Plasmodium vivax remains an important cause of malaria in South America and the Asia-Pacific. Naturally acquired antibody responses against multiple P. vivax proteins have been described in numerous countries, however, direct comparison of these responses has been difficult with different methodologies employed. We measured antibody responses against 307 P. vivax proteins at the time of P. vivax infection, and at 2–3 later time-points in three countries. We observed that seropositivity rates at the time of infection were highest in Thailand, followed by Brazil then PNG, reflecting the level of antigenic input. The majority of sero-reactive antigens in all sites induced short-lived antibody responses with estimated half-lives of less than 6 months, although there was a trend towards longer-lived responses in PNG children. Despite these differences, IgG seropositivity rates, magnitude and longevity were highly and significantly rank-correlated between the different regions, suggesting such features are reflective of the individual protein. [ABSTRACT FROM AUTHOR]

The abundance of Aedes mosquito species enabled widespread transmission of mosquito-borne chikungunya virus (CHIKV) and dengue virus (DENV) in Southeast Asia. Periodic seroprevalence surveys are therefore necessary to assess the viral burden in the population and the effectiveness of public health interventions. Since the current seroprevalence for CHIKV and DENV in Thailand are unknown, we evaluated evidence of past infection among Thais. Eight-hundred and thirty-five serum samples obtained from individuals living in central and southern Thailand were assessed for anti-CHIKV and anti-DENV IgG antibodies using commercial enzyme-linked immunosorbent assays. Overall, 26.8% (224/835) of individuals were seropositive for CHIKV, the majority of whom were also DENV-seropositive (91.1%, 204/224). Approximately half of all adults in their fifth decade of life had attained CHIKV seropositivity. Children under 15 years of age in southern Thailand were significantly more likely to be CHIKV-seropositive compared to those residing in central Thailand. In contrast, 79.2% (661/835) of Thais were DENV-seropositive, 30.9% (204/661) of whom also had antibodies to CHIKV. CHIKV/DENV dual seropositivity among Thais was 24.4% (204/835). The age-standardized seroprevalence for DENV was three times that of CHIKV (80.5% vs. 27.2%). Relatively high CHIKV seroprevalence among adults living in central Thailand revealed an under-recognized CHIKV burden in the region, while the low-to-moderate transmission intensity of DENV (seroprevalence <50% at 9 years) is expected to reduce the impact of DENV vaccination in Thailand. This most recent seroprevalence data provide serological baselines for two of the most common mosquito-borne viruses in this region. [ABSTRACT FROM AUTHOR]

Improved awareness of the hepatitis C virus (HCV) transmission has contributed to the overall decline in the HCV infection rate in some developing countries including Thailand. Chronic HCV infection in some rural Thai communities, however, presents a challenge in the efforts to treat and manage HCV-related diseases. Published and unpublished studies have suggested an unusually high incidence of HCV infection in a Thai province of Phetchabun compared to elsewhere in Thailand. To determine the magnitude of HCV infection and identify potential factors contributing to the higher rate of HCV infection in this province, we performed a population-based study in Phetchabun (n = 1667) and the neighboring Khon Kaen province (n = 1410) where HCV prevalence is much lower. Individuals between 30 and 64 years old completed detailed questionnaires designed to identify HCV risk factors and provided blood samples for anti-HCV antibody screening. The anti-HCV seropositive rates were 15.5% (259/1667) in Phetchabun and 3.6% (51/1410) in Khon Kaen. Positive samples were subsequently genotyped for HCV core gene sequence and assessed for the hepatitis B virus surface antigen (HBsAg) and human immunodeficiency virus antigen/antibody (HIV Ag/Ab). More individuals in Phetchabun possessed the combined presence of HBsAg (5.0%) and HIV Ag/Ab (0.4%) than those in Khon Kaen (3.9% HBsAg and 0.0% HIV Ag/Ab). While male gender, intravenous drug use (IVDU) and tattoos were significant HCV risk factors in both provinces (p <0.05), education less than high school and agriculture-related occupation were additionally associated with HCV in Phetchabun. HCV genotypes 6, 3, and 1 were identified in similar frequency in both provinces. We estimated that prevalence of HCV seropositivity and viremic carriers were higher in Phetchabun (143 and 111 per 1000) than in Khon Kaen (34 and 22 per 1000). Finally, we derived a simple risk factor-based scoring system as a useful preclinical tool to screen individuals at risk of chronic HCV infection prior to intervention. Knowledge gained from this study will assist in HCV screening and promote access to anti-viral treatment in high-risk groups. [ABSTRACT FROM AUTHOR]

Despite the high coverage of prophylactic vaccine against Bordetella pertussis infection in many countries for more than three decades, pertussis remains a common vaccine-preventable disease. Infections have been detected more commonly in countries using acellular pertussis vaccine in their Expanded Program of Immunization. Thailand implemented a routine infant immunization program with whole-cell pertussis vaccine in 1977, and since 1992, the national vaccine policy has offered a five-dose whole-cell pertussis vaccine for children given at the ages of 2, 4, 6, 18, and 48 months. This study aimed to investigate the seroprevalence of antibodies to pertussis toxin among healthy people across all ages to determine the level of whole-cell vaccine-induced immunity in the population, and to identify which age group should be targeted for a booster dose. The lowest seronegative rate and highest geometric mean concentrations were found in the 0–10 years age group, corresponding to their recent pertussis vaccination. The proportion of people with undetectable IgG level was prominent, starting after 11 years of age onwards. Now that a reduced-dose pertussis vaccine with fewer adverse effects is available, a booster dose during adolescence should be considered in order to reduce the incidence of pertussis disease. Further studies exploring how long the reduced-dose pertussis vaccine can provide protective immunity against pertussis disease when administered to adults and adolescents should also be performed. [ABSTRACT FROM AUTHOR]