Immunogenicity Induced by the Heptavalent Pneumococcal Conjugate Vaccine among Thai Children at High Risk for Severe Pneumococcal Disease

Abstract

Background: Streptococcus pneumoniae, an important pathogen worldwide, causes a wide range of diseases mostly in young children and older adults. The heptavalent pneumococcal conjugate vaccine (PCV7) that is composed of seven serotypes (4, 6B, 9V, 14, 18C, 19F, and 23F), was most prevalent in Thailand during this vaccination study. Aim: To determine immune responses to PCV7 among Thai children with high risk for invasive pneumococcal disease and whether children had specific antibody responses to related serotype 19A. Methods: The antibody-mediated opsonophagocytic killing assay (OPKA) was employed to detect the humoral immune response. One hundred and twenty-nine children (72 males and 57 females) were enrolled and divided into three groups according to age and vaccine dosages received: >12 months and received two vaccine doses, 7-12 months and received three vaccine doses, and 2-6 months and received four vaccine doses. These groups were then categorised into five groups based on disease: congenital heart disease, prematurity, chronic lung disease, haematological disease, and HIV. Serotype-specific antibody responses from children in all disease groups were comparatively analysed. Results: Low antibody responses to serotypes 9V and 23F, moderate responsive antibody titres to serotypes 4, 18C, 19A, and 19F, and high antibody responses to serotypes 6B and 14 were observed. Conclusion: These results suggest that children at high risk of invasive pneumococcal disease could develop functional antibody responses to some vaccine serotypes after PCV7 vaccination.