Identification of age-group reservoirs for persistent vaccine-type pneumococcal carriage in rural Gambia.

Abstract

Background: Although pneumococcal conjugate vaccines (PCVs) have been used widely in many low and middle-income countries, residual vaccine-type (VT) carriage persists in these settings. We examined the role of specific age groups in transmission and as reservoirs of VT pneumococcal infection in The Gambia.

Methods: Between January and November 2022, we conducted a nested, population-based, cross-sectional social contact and pneumococcal carriage survey in the Central and Upper River Regions of The Gambia. Participants completed questionnaires on carriage risk factors and social contacts. Nasopharyngeal swabs were collected from selected household members across all age groups. Streptococcus pneumoniae was isolated and serotyped using standard methods. We analysed matched contact and pneumococcal carriage data and estimated the proportions of VT carriage attributable to contact with different age groups.

Results: A total of 1638 participants were enrolled, of which 67% were children aged 0-14 years. Pneumococcal carriage prevalence was 59.6% (95% CI: 53.9 - 65.1%) in 0-4 year-olds and 36.1% (95% CI: 29.6 - 43.1%) in 5-14 year-olds. PCV13 VT carriage prevalence was not significantly different (10-13%) between these age groups. Among pneumococcal carriers, the proportion of VT carriage was significantly higher in 5- 14-year-olds [35.7% (95% CI: 25.9 - 46.9%)] compared to 0-4-year-olds [17.8% (95% CI: 13.9 - 22.6%, p-value < 0.001)]. The odds of VT carriage were 10% higher [AOR = 1.10, 95% CI: 1.01-1.20] for each additional physical contact with a child aged 10-14 years. We estimated that children aged 5-14 years contributed about 63% to the overall risk of exposure to VT pneumococci in the population.

Conclusions: In rural Gambia, school-aged children, particularly those aged 5-9 years, are the main drivers of VT pneumococcal transmission. In the context of high coverage of routine PCV vaccination in infants, this suggests waning PCV protection by school entry. A booster dose for children at school entry may support better control of VT circulation in the population.