Estimating the contribution of different age strata to vaccine serotype pneumococcal transmission in the pre vaccine era: a modelling study

BACKGROUND

Herd protection through interruption of transmission has contributed greatly to the impact of pneumococcal conjugate vaccines (PCVs) and may enable the use of cost-saving reduced dose schedules. To aid PCV age targeting to achieve herd protection, we estimated which population age groups contribute most to vaccine serotype (VT) pneumococcal transmission.

METHODS

We used transmission dynamic models to mirror pre-PCV epidemiology in England and Wales, Finland, Kilifi in Kenya and Nha Trang in Vietnam where data on carriage prevalence in infants, pre-school and school-aged children and adults as well as social contact patterns was available. We used Markov Chain Monte Carlo methods to fit the models and then extracted the per capita and population-based contribution of different age groups to VT transmission.

RESULTS

We estimated that in all settings, < 1-year-old infants cause very frequent secondary vaccine type pneumococcal infections per capita. However, 1-5-year-old children have the much higher contribution to the force of infection at 51% (28, 73), 40% (27, 59), 37% (28, 48) and 67% (41, 86) of the total infection pressure in E&W, Finland, Kilifi and Nha Trang, respectively. Unlike the other settings, school-aged children in Kilifi were the dominant source for VT infections with 42% (29, 54) of all infections caused. Similarly, we estimated that the main source of VT infections in infants are pre-school children and that in Kilifi 39% (28, 51) of VT infant infections stem from school-aged children whereas this was below 15% in the other settings.

CONCLUSION

Vaccine protection of pre-school children is key for PCV herd immunity. However, in high transmission settings, school-aged children may substantially contribute to transmission and likely have waned much of their PCV protection under currently recommended schedules.

Population based cohort study for pediatric infectious diseases research in Vietnam

A population-based cohort study on pediatric infectious diseases was established at Khanh Hoa Province, central Vietnam in 2006, to determine the etiology and risk factors for severe pediatric infectious diseases (SPID) such as acute respiratory infection (ARI), diarrhea and dengue which are the major causes of under 5 mortality. A population census survey was conducted in Nha-Trang and Ninh-Hoa to collect demographic, social-behavioral data and disease burden on SPID. The study site covered a population of 353,525 residing in 75,826 households with 24,781 children less than 5 years. Hospital databases from two hospitals covering the region were obtained. Linking the census and hospital databases, we were able to investigate on a variety of SPID such as environmental tobacco smoking exposure and increased risked of pediatric pneumonia hospitalization, population density, water supply and risk of dengue fever and animal livestock and risk of hospitalized diarrhea. To determine incidence, viral etiology and risk factors for pediatric ARI/pneumonia, we setup a population based prospective hospitalized Pediatric ARI surveillance at Khanh Hoa General Hospital, Nha-Trang in February 2007. The study has revealed RSV, rhinovirus and influenza A as major viral pathogens, role of multiple viral infection and its interaction with bacteria in the development of pneumonia. In addition, we are also conducting a birth cohort study to investigate the incidence of congenital infection and its impact on physical-neurological development, and role of host genetic polymorphism on SPID hospitalization in Vietnam. Population mobility, high cost of regular census update and low mortality are the challenges.

Incidence of radiologically-confirmed pneumonia and Haemophilus influenzae type b carriage before Haemophilus influenzae type b conjugate vaccine introduction in Central Vietnam

OBJECTIVES

To determine the incidence of radiologically-confirmed pneumonia (RCP) and Haemophilus influenzae type b (Hib) carriage in central Vietnam as a baseline data before Hib conjugate vaccine introduction.

STUDY DESIGN

In the context of ongoing population-based prospective, hospitalized acute respiratory infection surveillance study, a cross-sectional Hib carriage study was conducted among 1000 children < 5 years of age living in NhaTrang, Vietnam in June 2010, 1 month before the nationwide introduction of Hib conjugate vaccine in Vietnam.

RESULTS

The incidence of RCP hospitalizations among children < 5 years of age was 3.3 per 1000 children. The highest incidence was observed among children 12-23 month age group (8.3 per 1000). Haemophilus influenzae carriage was detected in 37% of the children and Hib carriage rate was 3%. Eighty-two percent of the Haemophilus influenzae had TEM β-lactamase resistance gene. The presence of 6 or more family members was associated with an increased rate of Hib carriage (P = .04).

CONCLUSIONS

Incidence of RCP and Hib carriage in this cross-sectional survey are lower compared with other studies. Continued surveillance for invasive Hib disease and sequential Hib carriage surveys are needed to support future assessments of the impact of Hib conjugate vaccine in Vietnam.

Animal livestock and the risk of hospitalized diarrhoea in children under 5 years in Vietnam

OBJECTIVE

To investigate the association between environmental exposure to livestock and incidence of diarrhoea among Vietnamese children.

METHODS

A population-based cohort of 353,525 individuals, living in 75,828 households in Khanh Hoa Province, Vietnam, with baseline data covering geo-referenced information on demography, socio-economic status and household animals was investigated. Geographic information system was applied to calculate the density of livestock. The data were linked to hospitalized diarrhoea cases of children under 5 years recorded at two hospitals treating patients from the area as inpatients in the study area.

RESULTS

Overall, 3116 children with diarrhoea were hospitalized during the study period. The incidence of diarrhoea hospitalization was 60.8/1000 child-years. Male gender, age <2 years, higher number of household members and lack of tap water were significantly associated with an increased risk of diarrhoea. There was no evidence that ownership of livestock increased the risk of diarrhoea. In spatial analysis, we found no evidence that a high density of any animals was associated with an increased risk of diarrhoea.

CONCLUSIONS

Exposure to animals near or in households does not seem to constitute a major risk for diarrhoea in children under the age of 5 in Vietnam. Public health interventions to reduce childhood diarrhoea burden should focus on well-recognized causes such as sanitation, personal hygiene, access to adequate clean water supply and vaccination.

User fee exemption does not affect lower rates of hospital admission of girls in Vietnam

In many countries, girls have been reported to be less often admitted to hospital than boys. We studied the influence of socio-economic factors, education and access to health care on girls' and boys' admission rates for pneumonia, diarrhoea and dengue fever in south-central Vietnam. We explored whether the user fee exemption for children under 6 years introduced in 2005 had an impact on girls' admission rates. In a cohort analysis, we used data from a large census in Khanh Hoa Province conducted in 2006, linked to hospital admission records at individual level. We further analysed a cross-sectional health care utilization survey in a sample of children reported ill at the census. There were 38 731 children under 6 years among a total census population of 353 891. Overall, girls under the age of 6 years were 29% less likely to be admitted to hospital than boys. The gender differences in admission rates in children under 6 years were similar for diarrhoea, pneumonia and dengue. None of the socio-economic and educational factors appeared to affect the gender difference. The user fee exemption starting from October 2005 had no impact on the girls/boys rate ratio of admission. In conclusion, the higher hospital admission rates of boys compared with girls in Vietnam are independent of socio-economic factors and user fees. Higher susceptibility of boys to severe disease could explain part of the gender gap, but profound cultural norms and beliefs may also have contributed to the findings.

Population density, water supply, and the risk of dengue fever in Vietnam: cohort study and spatial analysis

BACKGROUND

Aedes aegypti, the major vector of dengue viruses, often breeds in water storage containers used by households without tap water supply, and occurs in high numbers even in dense urban areas. We analysed the interaction between human population density and lack of tap water as a cause of dengue fever outbreaks with the aim of identifying geographic areas at highest risk.

METHODS AND FINDINGS

We conducted an individual-level cohort study in a population of 75,000 geo-referenced households in Vietnam over the course of two epidemics, on the basis of dengue hospital admissions (n = 3,013). We applied space-time scan statistics and mathematical models to confirm the findings. We identified a surprisingly narrow range of critical human population densities between around 3,000 to 7,000 people/km² prone to dengue outbreaks. In the study area, this population density was typical of villages and some peri-urban areas. Scan statistics showed that areas with a high population density or adequate water supply did not experience severe outbreaks. The risk of dengue was higher in rural than in urban areas, largely explained by lack of piped water supply, and in human population densities more often falling within the critical range. Mathematical modeling suggests that simple assumptions regarding area-level vector/host ratios may explain the occurrence of outbreaks.

CONCLUSIONS

Rural areas may contribute at least as much to the dissemination of dengue fever as cities. Improving water supply and vector control in areas with a human population density critical for dengue transmission could increase the efficiency of control efforts. Please see later in the article for the Editors' Summary.