Invasive Haemophilus influenzae Type b (Hib) Infection in a Fully Vaccinated Child: A Case Report

Background: Invasive Haemophilus influenzae type b (Hib) infection in children has become rare following the introduction of highly effective conjugate vaccines under national immunisation programmes. However, breakthrough invasive infections in fully immunised individuals can still occur. We report a case of invasive Hib infection presenting as epiglottitis and bacteraemia in a fully vaccinated 5-year-old boy in regional Queensland, Australia. Case presentation: The child, with no history of immunodeficiency, presented with a 3-day history of fever, progressive throat pain and reduced oral intake. Subsequent investigations revealed leukocytosis with left shift, markedly elevated C-reactive protein, and radiographic features consistent with epiglottitis. Blood culture grew H. influenzae type b. He was treated with intravenous cefotaxime and made a full recovery without complications. Immunological evaluation demonstrated Hib-specific IgG levels consistent with prior vaccinations, with normal immunoglobulin and lymphocyte profiles supporting the absence of immunodeficiency. Whole-genome sequencing of the isolate identified sequence type 6, a known circulating strain, and duplication of the capsule (cap-b) locus which has been associated with increased capsular polysaccharide production and reduced susceptibility to immune-mediated clearance. Conclusions: This case demonstrates that invasive Hib disease can occur in fully vaccinated, immunocompetent individuals and highlights the need for continued clinical vigilance. Pathogen-related factors, such as cap-b locus duplication, may reduce the effectiveness of the immune response. Despite this, immunisation can still confer partial protection, potentially contributing to the relatively mild clinical course. Ongoing surveillance and detailed microbiological investigation are essential to better understand and monitor vaccine breakthrough infections.