[Show abstract][Hide abstract] ABSTRACT: Background:
The mean age of dengue has been increasing in some but not all countries. We sought to determine the incidence of dengue virus (DENV) infection in adults and children in a prospective cohort study in the Philippines where dengue is hyperendemic.
A prospective cohort of subjects ≥6 months old in Cebu City, Philippines, underwent active community-based surveillance for acute febrile illnesses by weekly contact. Fever history within the prior seven days was evaluated with an acute illness visit followed by 2, 5, and 8-day, and 3-week convalescent visits. Blood was collected at the acute and 3-week visits. Scheduled visits took place at enrolment and 12 months that included blood collections. Acute samples were tested by DENV PCR and acute/convalescent samples by DENV IgM/IgG ELISA to identify symptomatic infections. Enrolment and 12-month samples were tested by DENV hemagglutination inhibition (HAI) assay to identify subclinical infections. Of 1,008 enrolled subjects, 854 completed all study activities at 12 months per-protocol undergoing 868 person-years of surveillance. The incidence of symptomatic and subclinical infections was 1.62 and 7.03 per 100 person-years, respectively. However, in subjects >15 years old, only one symptomatic infection occurred whereas 27 subclinical infections were identified. DENV HAI seroprevalence increased sharply with age with baseline multitypic HAIs associated with fewer symptomatic infections. Using a catalytic model, the historical infection rate among dengue naïve individuals was estimated to be high at 11-22%/year.
In this hyperendemic area with high seroprevalence of multitypic DENV HAIs in adults, symptomatic dengue rarely occurred in individuals older than 15 years. Our findings demonstrate that dengue is primarily a pediatric disease in areas with high force of infection. However, the average age of dengue could increase if force of infection decreases over time, as is occurring in some hyperendemic countries such as Thailand.
[Show abstract][Hide abstract] ABSTRACT: The immune response to dengue virus (DENV) infection is complex and not fully understood. Using longitudinal data from 181
children with dengue in Thailand, followed for up to three years, we describe neutralizing antibody kinetics following symptomatic
DENV infection. We observed that antibody titers varied by serotype, homotypic versus heterotypic responses, and primary versus
post-primary infections. The rates of change in antibody titers over time varied between primary and post-primary responses.
For primary infections, titers increased from convalescence to 6 months. By comparing homotypic versus heterotypic antibody
titers we saw an increase in type-specificity from convalescence to 6 months for primary DENV3 infections but not primary
DENV1 infections. In post-primary cases, there was a decrease in titers from convalescence up until 6 months post infection.
From one year after both primary and post-primary infections there was evidence of increases in antibody titers, with greater
increases in those with lower titers, suggesting boosting of antibody due to infection, and that higher levels of neutralizing
antibody may be more likely to infer a sterilizing immune response. These findings may help to model virus transmission dynamics
and provide baseline data to support the development of vaccines and therapeutics.
Full-text · Article · Dec 2015 · The Journal of Infectious Diseases
[Show abstract][Hide abstract] ABSTRACT: Background:
Infants born to dengue immune mothers acquire maternal antibodies to dengue. These antibodies, though initially protective, decline during the first year of life to levels thought to be disease enhancing, before reaching undetectable levels. Infants have long been studied to understand the interaction between infection and disease on an individual level.
Considering infants (cases <1 year old) as a unique group, we analyzed serotype specific dengue case data from patients admitted to a pediatric hospital in Bangkok, Thailand. We show differences in the propensity of serotypes to cause disease in individuals with dengue antibodies (infants and post-primary cases) and in individuals without dengue antibodies (primary cases). The mean age of infant cases differed among serotypes, consistent with previously observed differential waning of maternal antibody titers by serotype. We show that trends over time in epidemiology of infant cases are consistent with those observed in the whole population, and therefore with trends in the force of infection.
Infants with dengue are informative about the interaction between antibody and the dengue serotypes, confirming that in this population DENV-2 and DENV-4 almost exclusively cause disease in the presence of dengue antibody despite infections occurring in others. We also observe differences between the serotypes in the mean age in infant cases, informative about the interaction between waning immunity and disease for the different serotypes in infants. In addition, we show that the mean age of infant cases over time is informative about transmission in the whole population. Therefore, ongoing surveillance for dengue in infants could provide useful insights into dengue epidemiology, particularly after the introduction of a dengue vaccine targeting adults and older children.
[Show abstract][Hide abstract] ABSTRACT: The effect of prior dengue virus (DENV) exposure on subsequent heterologous infection can be beneficial or detrimental depending on many factors including timing of infection. We sought to evaluate this effect by examining a large database of DENV infections captured by both active and passive surveillance encompassing a wide clinical spectrum of disease.
We evaluated datasets from 17 years of hospital-based passive surveillance and nine years of cohort studies, including clinical and subclinical DENV infections, to assess the outcomes of sequential heterologous infections. Chi square or Fisher's exact test was used to compare proportions of infection outcomes such as disease severity; ANOVA was used for continuous variables. Multivariate logistic regression was used to assess risk factors for infection outcomes.
Of 38,740 DENV infections, two or more infections were detected in 502 individuals; 14 had three infections. The mean ages at the time of the first and second detected infections were 7.6 ± 3.0 and 11.2 ± 3.0 years. The shortest time between sequential infections was 66 days. A longer time interval between sequential infections was associated with dengue hemorrhagic fever (DHF) in the second detected infection (OR 1.3, 95%CI 1.2-1.4). All possible sequential serotype pairs were observed among 201 subjects with DHF at the second detected infection, except DENV-4 followed by DENV-3. Among DENV infections detected in cohort subjects by active study surveillance and subsequent non-study hospital-based passive surveillance, hospitalization at the first detected infection increased the likelihood of hospitalization at the second detected infection.
Increasing time between sequential DENV infections was associated with greater severity of the second detected infection, supporting the role of heterotypic immunity in both protection and enhancement. Hospitalization was positively associated between the first and second detected infections, suggesting a possible predisposition in some individuals to more severe dengue disease.
Full-text · Article · Dec 2015 · BMC Public Health
[Show abstract][Hide abstract] ABSTRACT: Killer immunoglobulin-like receptors (KIRs) interact with HLA class I ligands and play a key role in the regulation and activation of NK cells. The functional importance of KIR-HLA interactions has been demonstrated for a number of chronic viral infections, but to date only a few studies have been performed in the context of acute self-limited viral infections. During our investigation of CD8(+) T cell responses to a conserved HLA-B57-restricted epitope derived from dengue virus (DENV) non-structural protein-1 (NS1), we observed substantial binding of the tetrameric complex to non-T/non-B lymphocytes in PBMC from a long-standing clinical cohort in Thailand. We confirmed binding of the NS1 tetramer to CD56(dim) NK cells, which are known to express KIRs. Using depletion studies and KIR-transfected cell lines, we further demonstrated that the NS1 tetramer bound the inhibitory receptor KIR3DL1. Phenotypic analysis of PBMC from HLA-B57(+) subjects with acute DENV infection revealed marked activation of NS1 tetramer-binding NK cells around the time of defervescence in subjects with severe dengue disease. Collectively, our findings indicate that subsets of NK cells are activated relatively late in the course of acute DENV illness and reveal a possible role for specific KIR-HLA interactions in the modulation of disease outcomes. This article is protected by copyright. All rights reserved.
No preview · Article · Oct 2015 · Clinical & Experimental Immunology
[Show abstract][Hide abstract] ABSTRACT: Proper understanding of the long-term epidemiology of chikungunya has been hampered by poor surveillance. Outbreak years are
unpredictable and cases often misdiagnosed. Here we analyzed age-specific data from two serological studies (from 1973 and
2012) in Cebu, Philippines, to reconstruct both the annual probability of infection and population-level immunity over a 60-year
period (1952-2012). We also explored whether seroconversions during 2012-2013 were spatially clustered. Our models identified
four discrete outbreaks separated by an average delay of 17 years. On average, 23% (95% CI: 16%-37%) of the susceptible population
was infected per outbreak with >50% of the entire population remaining susceptible at any point. Participants who seroconverted
during 2012-2013 were clustered at distances <230m suggesting focal transmission. Large-scale outbreaks of chikungunya did
not result in sustained multi-year transmission. Nevertheless, we estimate >350,000 infections were missed by surveillance
systems. Serological studies could supplement surveillance to provide important insights on pathogen circulation.
Full-text · Article · Sep 2015 · The Journal of Infectious Diseases
[Show abstract][Hide abstract] ABSTRACT: Chikungunya virus (CHIKV) is a globally re-emerging arbovirus for which previous studies have indicated the majority of infections result in symptomatic febrile illness. We sought to characterize the proportion of subclinical and symptomatic CHIKV infections in a prospective cohort study in a country with known CHIKV circulation.
A prospective longitudinal cohort of subjects ≥6 months old underwent community-based active surveillance for acute febrile illness in Cebu City, Philippines from 2012-13. Subjects with fever history were clinically evaluated at acute, 2, 5, and 8 day visits, and at a 3-week convalescent visit. Blood was collected at the acute and 3-week convalescent visits. Symptomatic CHIKV infections were identified by positive CHIKV PCR in acute blood samples and/or CHIKV IgM/IgG ELISA seroconversion in paired acute/convalescent samples. Enrollment and 12-month blood samples underwent plaque reduction neutralization test (PRNT) using CHIKV attenuated strain 181/clone25. Subclinical CHIKV infections were identified by ≥8-fold rise from a baseline enrollment PRNT titer <10 without symptomatic infection detected during the intervening surveillance period. Selected CHIKV PCR-positive samples underwent viral isolation and envelope protein-1 gene sequencing. Of 853 subjects who completed all study procedures at 12 months, 19 symptomatic infections (2.19 per 100 person-years) and 87 subclinical infections (10.03 per 100 person-years) occurred. The ratio of subclinical-to-symptomatic infections was 4.6:1 varying with age from 2:1 in 6 month-5 year olds to 12:1 in those >50 years old. Baseline CHIKV PRNT titer ≥10 was associated with 100% (95%CI: 46.1, 100.0) protection from symptomatic CHIKV infection. Phylogenetic analysis demonstrated Asian genotype closely related to strains from Asia and the Caribbean.
Subclinical infections accounted for a majority of total CHIKV infections. A positive baseline CHIKV PRNT titer was associated with protection from symptomatic CHIKV infection. These findings have implications for assessing disease burden, understanding virus transmission, and supporting vaccine development.
[Show abstract][Hide abstract] ABSTRACT: In Viet Nam, an inactivated, mouse brain-derived vaccine for Japanese encephalitis (JE) has been given exclusively to ≤5 years old children in 3 paediatric doses since 1997. However, JE incidence remained high, especially among children aged 5-9 years. We conducted a model JE immunization programme to assess the feasibility and impact of JE vaccine administered to 1-9 year(s) children in 3 standard-dose regimen: paediatric doses for children aged <3 years and adult doses for those aged ≥3 years. Of the targeted children, 96.2% were immunized with ≥2 doses of the vaccine. Compared to the national immunization programme, JE incidence rate declined sharply in districts with the model programme (11.32 to 0.87 per 100,000 in pre-versus post-vaccination period). The rate of reduction was most significant in the 5-9 years age-group. We recommend a policy change to include 5-9 years old children in the catch-up immunization campaign and administer a 4th dose to those aged 5-9 years, who had received 3 doses of the vaccine during the first 2-3 years of life.
Full-text · Article · Mar 2015 · Journal of Health Population and Nutrition
[Show abstract][Hide abstract] ABSTRACT: Background:
AFRIMS longitudinal dengue surveillance in Thailand depends on the nested RT-PCR and the dengue IgM/IgG ELISA.
To examine and improve the sensitivity of the nested RT-PCR using a panel of archived samples collected during dengue surveillance.
A retrospective analysis of 16,454 dengue IgM/IgG ELISA positive cases collected between 2000 and 2013 was done to investigate the sensitivity of the nested RT-PCR. From these cases, 318 acute serum specimens or extracted RNA, previously found to be negative by the nested RT-PCR, were tested using TaqMan real-time RT-PCR (TaqMan rRT-PCR). To improve the sensitivity of nested RT-PCR, we designed a new primer based on nucleotide sequences from contemporary strains found to be positive by the TaqMan rRT-PCR. Sensitivity of the new nested PCR was calculated using a panel of 87 samples collected during 2011-2013.
Results and conclusion:
The percentage of dengue IgM/IgG ELISA positive cases that were negative by the nested RT-PCR varied from 17% to 42% for all serotypes depending on the year. Using TaqMan rRT-PCR, dengue RNA was detected in 194 (61%) of the 318 acute sera or extracted RNA previously found to be negative by the nested RT-PCR. The newly designed DENV-1 specific primer increased the sensitivity of DENV-1 detection by the nested RT-PCR from 48% to 88%, and of all 4 serotypes from 73% to 87%. These findings demonstrate the impact of genetic diversity and signal erosion on the sensitivity of PCR-based methods.
No preview · Article · Feb 2015 · Journal of Clinical Virology
[Show abstract][Hide abstract] ABSTRACT: The WHO 'Global Strategy for Dengue Prevention and Control, 2012-2020' addresses the growing need for the treatment of dengue, and targets a 25% reduction in morbidity and 50% in mortality (using 2010 estimates as baseline). Achieving these goals requires future dengue prevention strategies that will employ both potential vaccines and sustainable vector-control measures. Maternally transferred dengue antibody is an important factor in determining the optimal age for dengue vaccination.
To estimate the seroprevalence of dengue antibodies among mothers living in an area of high endemicity - Ban Pong, Ratchaburi Province - and to assess maternal dengue antibodies transferred to cord blood.
A cross-sectional study was conducted with 141 pregnant women who delivered at Ban Pong Hospital, Ratchaburi, Thailand. Maternal-cord paired sera were tested for dengue neutralizing (NT) antibody by PRNT50 assay. A ratio of ≥ 1:10 NT titer to dengue serotype was considered seropositive.
Most mothers (137/141, 97.2%) had NT antibodies to at least one dengue serotype in their sera. At birth, the proportion of cord sera with NT antibodies to DEN-1, DEN-2, DEN-3, and DEN-4, were high and similar to the sera of their mothers, at 93.6%, 97.2%, 97.9%, and 92.2%, respectively. The dengue geometric mean titers (GMT) in cord blood were significantly higher than the maternal antibodies (p<0.001): highest in DEN-2, followed by DEN-3, and then DEN-1. The GMT of DEN-4 was the lowest among all four serotypes.
Dengue infection is highly prevalent among pregnant women in this dengue-endemic area. Most of the cord blood had transferred dengue antibodies, which may have an impact on the disease burden in this population.
[Show abstract][Hide abstract] ABSTRACT: Background
Long-term homologous and temporary heterologous protection from dengue virus (DENV) infection may be mediated by neutralizing antibodies. However, neutralizing antibody titers (NTs) have not been clearly associated with protection from infection.
Data from two geographic cluster studies conducted in Kamphaeng Phet, Thailand were used for this analysis. In the first study (2004–2007), cluster investigations of 100-meter radius were triggered by DENV-infected index cases from a concurrent prospective cohort. Subjects between 6 months and 15 years old were evaluated for DENV infection at days 0 and 15 by DENV PCR and IgM ELISA. In the second study (2009–2012), clusters of 200-meter radius were triggered by DENV-infected index cases admitted to the provincial hospital. Subjects of any age ≥6 months were evaluated for DENV infection at days 0 and 14. In both studies, subjects who were DENV PCR positive at day 14/15 were considered to have been “susceptible” on day 0. Comparison subjects from houses in which someone had documented DENV infection, but the subject remained DENV negative at days 0 and 14/15, were considered “non-susceptible.” Day 0 samples were presumed to be from just before virus exposure, and underwent plaque reduction neutralization testing (PRNT). Seventeen “susceptible” (six DENV-1, five DENV-2, and six DENV-4), and 32 “non-susceptible” (13 exposed to DENV-1, 10 DENV-2, and 9 DENV-4) subjects were evaluated. Comparing subjects exposed to the same serotype, receiver operating characteristic (ROC) curves identified homotypic PRNT titers of 11, 323 and 16 for DENV-1, -2 and -4, respectively, to differentiate “susceptible” from “non-susceptible” subjects.
PRNT titers were associated with protection from infection by DENV-1, -2 and -4. Protective NTs appeared to be serotype-dependent and may be higher for DENV-2 than other serotypes. These findings are relevant for both dengue epidemiology studies and vaccine development efforts.
[Show abstract][Hide abstract] ABSTRACT: Background
Currently, no dengue NS1 detection kit has regulatory approval for the diagnosis of acute dengue fever. Here we report the sensitivity and specificity of the InBios DEN Detect NS1 ELISA using a panel of well characterized human acute fever serum specimens.
The InBios DENV Detect NS1 ELISA was tested using a panel composed of 334 serum specimens collected from acute febrile patients seeking care in a Bangkok hospital in 2010 and 2011. Of these patients, 314 were found to have acute dengue by either RT-PCR and/or anti-dengue IgM/IgG ELISA. Alongside the InBios NS1 ELISA kit, we compared the performance characteristics of the BioRad Platelia NS1 antigen kit. The InBios NS1 ELISA Ag kit had a higher overall sensitivity (86% vs 72.8%) but equal specificity (100%) compared to the BioRad Platelia kit. The serological status of the patient significantly influenced the outcome. In primary infections, the InBios NS1 kit demonstrated a higher sensitivity (98.8%) than in secondary infections (83.5%). We found significant variation in the sensitivity of the InBios NS1 ELISA kit depending on the serotype of the dengue virus and also found decreasing sensitivity the longer after the onset of illness, showing 100% sensitivity early during illness, but dropping below 50% by Day 7.
The InBios NS1 ELISA kit demonstrated high accuracy when compared to the initial clinical diagnosis with greater than 85% agreement when patients were clinically diagnosed with dengue illness. Results presented here suggest the accurate detection of circulating dengue NS1 by the InBios DENV Detect NS1 ELISA can provide clinicians with a useful tool for diagnosis of early dengue infections.
[Show abstract][Hide abstract] ABSTRACT: Dengue is endemic to the rural province of Kamphaeng Phet, Northern Thailand. A decade of prospective cohort studies has provided important insights into the dengue viruses and their generated disease. However, as elsewhere, spatial dynamics of the pathogen remain poorly understood. In particular, the spatial scale of transmission and the scale of clustering are poorly characterized. This information is critical for effective deployment of spatially targeted interventions and for understanding the mechanisms that drive the dispersal of the virus.
[Show abstract][Hide abstract] ABSTRACT: Accurate determination of neutralization antibody titers supports epidemiological studies of dengue virus transmission and vaccine trials. Neutralization titers measured using the plaque reduction neutralization test (PRNT) are believed to provide a key measure of immunity to dengue viruses, however, the assay's variability is poorly understood, making it difficult to interpret the significance of any assay reading. In addition there is limited standardization of the neutralization evaluation point or statistical model used to estimate titers across laboratories, with little understanding of the optimum approach.
[Show abstract][Hide abstract] ABSTRACT: Safety and immunogenicity of two formulations of a live-attenuated tetravalent dengue virus (TDEN) vaccine produced using rederived master seeds from a precursor vaccine were tested against a placebo control in a phase II, randomized, double blind trial (NCT00370682). Two doses were administered 6 months apart to 120 healthy, predominantly flavivirus-primed adults (87.5% and 97.5% in the two vaccine groups and 92.5% in the placebo group). Symptoms and signs reported after vaccination were mild to moderate and transient. There were no vaccine-related serious adverse events or dengue cases reported. Asymptomatic, low-level viremia (dengue virus type 2 [DENV-2], DENV-3, or DENV-4) was detected in 5 of 80 vaccine recipients. One placebo recipient developed a subclinical natural DENV-1 infection. All flavivirus-unprimed subjects and at least 97.1% of flavivirus-primed subjects were seropositive to antibodies against all four DENV types 1 and 3 months post-TDEN dose 2. The TDEN vaccine was immunogenic with an acceptable safety profile in flavivirus-primed adults.
Full-text · Article · May 2014 · The American journal of tropical medicine and hygiene
[Show abstract][Hide abstract] ABSTRACT: The Indian Ocean chikungunya epidemic re-emerged in Thailand in August 2008. Forty-five adults with laboratory-confirmed chikungunya in Songkhla province, Thailand were clinically assessed and serially bled throughout the acute and convalescent phase of the disease. Patient symptoms, antibody responses, and viral kinetics were evaluated using observational assessments, polymerase chain reaction (PCR), and serological assays. All subjects experienced joint pain with 42 (93%) involving multiple joints; the interphalangeal most commonly affected in 91% of the subjects. The mean duration of joint pain was 5.8 days, 11 (25%) experiencing discomfort through the duration of the study. Rash was observed in 37 (82%) subjects a mean 3.5 days post onset of symptoms. Patents were positive by PCR for a mean of 5.9 days with sustained peak viral load through Day 5. The IgM antibodies appeared on Day 4 and peaked at Day 7 and IgG antibodies first appeared at Day 5 and rose steadily through Day 24.
Full-text · Article · Feb 2014 · The American journal of tropical medicine and hygiene
[Show abstract][Hide abstract] ABSTRACT: Dengue virus has traditionally caused substantial morbidity and mortality among children less than 15 years of age in Southeast
Asia. Over the last 2 decades, a significant increase in the mean age of cases has been reported, and a once pediatric disease
now causes substantial burden among the adult population. An age-stratified serological study (n = 1,736) was conducted in 2010 among schoolchildren in the Mueang Rayong district of Thailand, where a similar study had
been conducted in 1980/1981. Serotype-specific forces of infection (λ(t)) and basic reproductive numbers (R0) of dengue were estimated for the periods 1969–1980 and 1993–2010. Despite a significant increase in the age at exposure
and a decrease in λ(t) from 0.038/year to 0.019/year, R0 changed only from 3.3 to 3.2. Significant heterogeneity was observed across subdistricts and schools, with R0 ranging between 1.7 and 6.8. These findings are consistent with the idea that the observed age shift might be a consequence
of the demographic transition in Thailand. Changes in critical vaccination fractions, estimated by using R0, have not accompanied the increase in age at exposure. These results have implications for dengue control interventions because
multiple countries in Southeast Asia are undergoing similar demographic transitions. It is likely that dengue will never again
be a disease exclusively of children.
Full-text · Article · Nov 2013 · American journal of epidemiology
[Show abstract][Hide abstract] ABSTRACT: A four-year longitudinal cohort and geographic cluster study in rural Thailand was conducted to characterize the clinical spectrum of dengue virus (DENV) infection. Symptomatic DENV infections in the cohort were detected by active school absence-based surveillance that triggered cluster investigations around ill cohort children. Data from 189 cohort children with symptomatic DENV infection and 126 contact children in the clusters with DENV infection were analyzed. Of infected contacts, only 19% were asymptomatic; 81% were symptomatic, but only 65.9% reported fever. Symptom-based case definitions were unreliable for diagnosis. Symptomatic infections in contacts were milder with lower DENV RNA levels than the cohort. Infections in contacts with fever history were more likely to have detectable DENV RNA than infections without fever history. Mild infections identified by cluster investigations account for a major proportion of all DENV infections. These findings are relevant for disease burden assessments, transmission modeling, and determination of vaccine impact.
No preview · Article · Oct 2013 · The American journal of tropical medicine and hygiene