Recurrent invasive pneumococcal disease in children--host factors and vaccination response

Streptococcus pneumoniae is still a leading cause of septicaemia, pneumonia and meningitis in young children world-wide with over half a million children dying annually from pneumococcal disease.  Some children are prone to repeated episodes of invasive pneumococcal disease (IPD) because of an underlying predisposing disease. Recurrent IPD (rIPD) is a rarity and published reports on rIPD are limited by having few children included, selected groups of patients or short follow-up periods. Deficiencies in the innate or adaptive immune system have been described in children with rIPD, but the frequency of immunodeficiency among such patients is unknown. The aim of this PhD thesis was to examine paediatric cases of laboratory-confirmed rIPD, over a 33-year period in Denmark, to determine risk factors and study aspects of the immunological background for this problem in children. In October 2007, a seven-valent pneumococcal conjugate vaccine (PCV7) was implemented in the Danish infant immunization programme. An additional aim of the thesis was to examine the impact of vaccination on a population level, following the first three years of general PCV7 vaccination in Denmark. The thesis consists of three papers, which are all directly or indirectly based on data retrieved from the National Streptococcus Pneumoniae Registry. This registry is nationwide and dates back to 1938. The registry contains data from all laboratory-confirmed cases of IPD in Denmark and is continually updated for national surveillance. In Paper 1, we conducted a 33-year retrospective nationwide study of paediatric rIPD. By using data from the National Streptococcus Pneumoniae Registry combined with clinical data from hospital records, we could describe one of the largest known cohorts of children (n:59) with rIPD . We covered epidemiological, microbiological, and clinical features of this clinical entity. Of all children experiencing rIPD, 47% had a known predisposing underlying disease at the time of the rIPD. Most common was immune deficiency due to transplantation. In 19% the episode of rIPD was the clinical manifestation that subsequently led to a diagnosis of an underlying disease. Finally, in 31% of the children no underlying disease was detected. Paper 2 covers data from a follow-up of the cohort of children described in Paper 1. Of this unselected cohort of rIPD, all children without an obvious underlying disease predisposing to pneumococcal disease (such as malignancy, HIV or cerebrospinal-fluid leakage) were invited to participate in the study by undergoing a thorough immunological evaluation. Basic immunological parameters including activity of complement-pathways and T-, B-, NK-cell count were examined in the children and their families. Furthermore, B-cell function including antibody response to polysaccharide-based pneumococcal vaccination and somatic hypermutation was evaluated. Toll like receptor (TLR) signalling function was evaluated in a functional assay. When children with classical risk factors for IPD were excluded, 15 individuals were eligible. Of whom, sex (40%) children with complement C2 deficiency were identified. Moreover, impaired vaccination response was found in six children: three with concurrent C2 deficiency and three with no other immune abnormality. One patient with a severe TLR signalling dysfunction was diagnosed. In Paper 3, we aimed to assess the impact of PCV7 in Denmark following the first three years of infant immunization. By comparing age-specific disease incidences of IPD in the pre-PCV7 (years 2000-2007) and the PCV7 periods (years 2008-2010) we sought to assess direct and indirect effects on incidence of IPD. In addition, changes in pneumococcal serotype distribution and IPD-related mortality were assessed. We documented a marked decline in the incidence of IPD in both vaccinated and non-vaccinated age groups. The overall incidence of IPD among children aged 0-5 years declined from 26.7 to 16.3 cases per 100,000 (IRR 0.58; 95% confidence interval (CI) [0.48-0.69]). A minor but statistically significant increase in the incidence of IPD due to non-vaccine type IPD was observed in both vaccinated and non-vaccinated groups, but with predominance of serotypes covered by the higher valence pneumococcal conjugate vaccines. This thesis confirms the existing knowledge on underlying diseases predisposing children to IPD, such as cerebrospinal fluid leakage, congenital heart disease and malignant diseases. Our findings support the notion that rIPD in a child should prompt a thorough search for an underlying disease. Moreover, our results underline that rIPD in a child without a known predisposing disease is a major pointer towards primary immune deficiency, such as complement deficiency and B cell dysfunction. This is in line with the guidelines put forward by various specialist committees. Finally, we reported data from the first three years of PCV7 vaccination in the Danish immunization programme, suggesting that the vaccine is effective against all serotypes included in the vaccine when administered in a 2+1 schedule.