Emerging Invasive Haemophilus influenzae Type C Infection in a Hib-vaccinated Child: Implications for Surveillance

To the Editors: Invasive Haemophilus influenzae disease has significantly decreased worldwide following widespread H. influenzae type B (Hib) vaccination.1 However, infections caused by non-type B serotypes are increasing2 and are not covered by current vaccines. Although rarely documented, invasive H. influenzae type C (Hic) can progress quickly and cause severe illness even in healthy, fully vaccinated children. To our knowledge, no well-documented cases have been reported in Japan. We describe a child with invasive Hic presenting as orbital cellulitis and subperiosteal abscess, highlighting the clinical severity, diagnostic difficulties and importance of continued surveillance of non-type B serotypes. A previously healthy 20-month-old girl, fully immunized with 4 Hib doses, developed fever and nasal discharge, followed by rapid swelling of the left eyelid the next morning. She had no history of immunodeficiency, recurrent infections, or international travel. Contrast-enhanced computed tomography within 24 hours revealed a 20 × 30 mm orbital abscess with bilateral ethmoid sinusitis. On admission, she exhibited eyelid erythema and swelling, conjunctival hyperemia, impaired ocular movement and postnasal drip, while otoscopic findings were normal. Blood tests revealed an elevated white blood cell count and C-reactive protein level of 22,100/μL and 12.1 mg/dL, respectively, whereas immunological screening was unremarkable. Magnetic resonance imaging confirmed a subperiosteal orbital abscess (Fig. 1A, B). On hospital day 2, she underwent incision and drainage and was started on empiric meropenem plus vancomycin, after which she became afebrile by day 3. Subsequently, H. influenzae was isolated from blood culture, and antibiotic therapy was de-escalated to cefazolin (Fig. 1C). Serotyping confirmed the strain as type C. After a 21-day course of antibiotics, she achieved full recovery without complications (Fig. 1D).FIGURE 1.: Radiological, microbiological, and clinical course of a child with invasive Haemophilus influenzae type C infection. A: Axial contrast-enhanced T1-weighted magnetic resonance image showing abscess formation in the left orbit with contrast enhancement, and (B) coronal short tau inversion recovery image showing abscess formation in the left orbit with deviation of the left eyeball. C: Antimicrobial susceptibility testing results. Antimicrobial susceptibility profile of Haemophilus influenzae type C identified from the blood culture. D: Clinical course and treatment after hospital admission. ABPC, ampicillin; ABPC/SBT, ampicillin/sulbactam; AMK, amikacin; AMPC, amoxicillin; CEZ, cefazolin; CRP, C-reactive protein; CTX, cefotaxime; FMOX, flomoxef; FOM, fosfomycin; MEPM, meropenem; PSL, prednisolone; VCM, vancomycin.Invasive Hic infections are uncommon but can lead to severe, rapidly progressive disease in otherwise healthy, fully vaccinated children. The orbital cellulitis and subperiosteal abscess in this case expand the known clinical spectrum and underscore the importance of considering non-type B serotypes in serious pediatric infections. Although only a few cases have been reported, they provide valuable context, including neonatal sepsis treated with ampicillin and gentamicin,3 respiratory distress in a term infant due to an ampicillin-sensitive strain4 and a lung abscess in a 5-year-old boy caused by an ampicillin-resistant isolate.5 Along with our case, these reports demonstrate the clinical heterogeneity of Hic, ranging from systemic to localized disease, and highlight the potential for favorable outcomes with prompt recognition and management. This case also illustrates important clinical lessons that invasive non-type B disease can occur despite full Hib immunization, emphasizing the need for continued vigilance. Early use of broad-spectrum antimicrobials, surgical drainage when necessary and subsequent de-escalation remain crucial. Given the presence of resistant isolates in previous reports, continued monitoring of antimicrobial susceptibility is necessary. Epidemiologically, non-type B serotypes are increasingly emerging as important pediatric pathogens in the post-Hib vaccine era, yet are outside current vaccine coverage. Reports from Asia remain limited, and our case provides valuable evidence from this underrepresented region. Enhanced surveillance and systematic reporting of invasive non-type B H. influenzae infections are essential for improving regional epidemiology and guiding future public health strategies. ACKNOWLEDGMENT The authors thank the family for permission to publish this case report.