What is Haemophilus influenzae?

Haemophilus influenzae is a Gram-negative coccobacillus — an obligate human commensal and pathogen that colonises the upper respiratory tract. Despite its confusingly misleading name, H. influenzae has absolutely no connection to influenza virus — it was erroneously identified as the cause of influenza during the 1890 pandemic and the name stuck. H. influenzae exists in encapsulated forms (six serotypes: a, b, c, d, e, f — with type b being by far the most virulent) and non-typeable forms (NTHi — no capsule). Before Hib vaccination, H. influenzae type b was the most common cause of bacterial meningitis in children under 5 globally.

What diseases does H. influenzae cause?

Invasive disease (from bloodstream spread): Meningitis (Hib — predominant in children; NTHi — increasingly in adults); epiglottitis (Hib — life-threatening airway obstruction in children, now rare due to vaccine); pneumonia with bacteraemia; septic arthritis; osteomyelitis; cellulitis (especially buccal/facial in young children). Non-invasive (mucosa only): COPD exacerbations (NTHi — the most common bacterial cause in adults); otitis media (NTHi — the most common bacterial cause in children); sinusitis; community-acquired pneumonia (NTHi — second most common bacterial cause after S. pneumoniae).

Why is non-typeable H. influenzae (NTHi) now more important?

Hib conjugate vaccination has been so successful that Hib disease is now extremely rare in well-vaccinated populations. However, the Hib vaccine does not protect against NTHi (which lacks the type b capsule). As Hib disease disappeared, the residual invasive H. influenzae disease is now predominantly NTHi in adults. NTHi invasive disease is increasing in: elderly adults (>65); immunocompromised patients; people with COPD or chronic lung disease; post-influenza bacterial superinfection. NTHi is also the most common bacterial cause of COPD exacerbations — a huge burden.

What antibiotic resistance is important in H. influenzae?

The main AMR mechanism: beta-lactamase production — approximately 30-40% of EU H. influenzae isolates produce TEM-1 beta-lactamase, rendering them resistant to ampicillin and amoxicillin. A minority also have beta-lactamase-negative ampicillin resistance (BLNAR) from altered penicillin-binding proteins — resistant to all aminopenicillins but susceptible to cephalosporins. Practical implications: Empirical treatment of invasive H. influenzae: ceftriaxone (covers all beta-lactamase producers). Oral treatment of NTHi respiratory infections (COPD exacerbation, CAP): amoxicillin-clavulanate (co-amoxiclav) if beta-lactamase likely; or doxycycline or a respiratory fluoroquinolone.

Is Hib vaccine still needed if the disease is so rare?

Yes — absolutely. The near-elimination of Hib disease IS the result of Hib vaccination, not a sign that vaccination is no longer needed. Hib vaccine creates herd immunity by eliminating nasopharyngeal carriage — children are the main reservoir. If vaccination rates fall, Hib will return. Several countries that discontinued Hib vaccination experienced resurgences within 5-10 years. Hib vaccine is on the WHO Essential Medicines List and recommended for all infants globally. It is one of the greatest public health achievements of the late 20th century.

How does GMJ News cover H. influenzae?

GMJ News covers the historical significance of Hib vaccination (95% reduction in childhood meningitis), the H. influenzae ≠ influenza naming confusion, the rise of NTHi invasive disease in adults, epiglottitis recognition, beta-lactamase resistance (ampicillin resistance), ceftriaxone treatment, rifampicin prophylaxis for contacts, and ECDC surveillance data. H. influenzae illustrates both the triumph of conjugate vaccines and the emergence of vaccine-escape pathogens (NTHi) — directly relevant to PHIG's vaccination and AMR programmes.

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Haemophilus influenzae Disease

GMJ News knowledge hub · last reviewed September 2026 · Georgian Medical Journal

Summary Statistics Glossary GMJ news FAQ Documents Organizations Research

Haemophilus influenzae — a Gram-negative coccobacillus with nothing to do with influenza virus (its misleading name derives from a 1890 pandemic misidentification) — caused catastrophic childhood meningitis, epiglottitis and pneumonia before the Hib conjugate vaccine was introduced, and while vaccine has reduced invasive H. influenzae type b (Hib) disease by over 95% globally, non-typeable H. influenzae (NTHi) — which the Hib vaccine does not protect against — is increasingly recognised as a major cause of respiratory infections in adults (COPD exacerbations, community-acquired pneumonia, otitis media) and invasive disease in the elderly and immunocompromised (ECDC). ECDC mandates surveillance of all invasive H. influenzae disease as a notifiable condition.

Key messages

Hib vaccine eliminated childhood meningitis — 95% reduction

Haemophilus influenzae type b (Hib) was the most common cause of bacterial meningitis in children under 5 globally before vaccination. Hib conjugate vaccine, introduced from the late 1980s, reduced invasive Hib disease by over 95% in countries with high coverage (WHO/ECDC).

H. influenzae ≠ influenza virus

A critical naming confusion: Haemophilus influenzae has absolutely nothing to do with influenza (flu) virus. It was erroneously named during the 1890 pandemic when it was mistakenly thought to be the causative agent. The bacterial genus Haemophilus and influenza virus are entirely unrelated.

NTHi — the non-vaccine serotype increasingly dominant

Non-typeable H. influenzae (NTHi — lacking a polysaccharide capsule) is NOT covered by the Hib vaccine. NTHi is now the dominant H. influenzae causing adult respiratory disease: COPD exacerbations, otitis media in children, community-acquired pneumonia, and increasingly invasive disease in elderly and immunocompromised adults.

Epiglottitis — now rare but classic emergency

Hib epiglottitis (life-threatening supraglottic airway obstruction from epiglottal swelling) was the classic paediatric airway emergency in the pre-vaccine era — a child leaning forward in a tripod position, drooling, unable to swallow. Now rare due to Hib vaccination but must be recognised when it occurs.

ECDC mandatory surveillance — all invasive Hib cases

ECDC mandates notification of all invasive H. influenzae disease (all serotypes) — including the growing burden of NTHi invasive disease in adults.

Ampicillin resistance — beta-lactamase producing H. influenzae (BLPHI)

Approximately 30-40% of H. influenzae isolates in Europe produce beta-lactamase — rendering them resistant to ampicillin and amoxicillin. Treatment of invasive H. influenzae requires ceftriaxone. Oral treatment of NTHi respiratory infections: amoxicillin-clavulanate (co-amoxiclav) if beta-lactamase suspected.

Key statistics

>95%

reduction in invasive Hib disease in countries with high Hib vaccine coverage

WHO/ECDC

NTHi

non-typeable H. influenzae now dominates adult invasive H. influenzae disease in EU

ECDC 2024

30-40%

of H. influenzae EU isolates are beta-lactamase producers (ampicillin resistant)

ECDC AMR data

Hib

conjugate vaccine in WHO EPI since 1998; reduces carriage (herd effect)

WHO

ECDC notifiable

mandatory invasive H. influenzae surveillance under EU Decision 2018/945

ECDC

1890

year H. influenzae wrongly named (Pfeiffer's bacillus during flu pandemic)

History

Invasive H. influenzae by serotype — EU/EEA ECDC data (pre and post-Hib vaccine era)

Source: ECDC. Hib disease virtually eliminated in vaccinated populations; NTHi now dominates invasive disease in adults.

Glossary of key terms

Haemophilus influenzae type b (Hib)
WHOThe encapsulated serotype b of H. influenzae — the most virulent human pathogen of the genus. The type b capsule (polyribosylribitol phosphate, PRP) enables complement evasion and systemic invasion. Pre-vaccination: the most common cause of bacterial meningitis in children under 5 globally; also caused epiglottitis, pneumonia, cellulitis, septic arthritis, osteomyelitis. Post-vaccination: extremely rare in vaccinated populations (vaccine effectiveness >95% against invasive Hib disease).

Hib conjugate vaccine (PRP-T)
WHO EMLPolysaccharide (PRP) conjugated to protein carrier (tetanus toxoid — PRP-T; or diphtheria toxoid CRM197 — PRP-OMP; meningococcal protein — PRP-OMP). Conjugation converts T-cell independent PRP response to T-cell dependent — enabling response in infants <2 years and immunological memory. Primary series: 2-3 doses at 2, 4, 6 months + optional booster at 12-18 months. Single dose is also partially protective. On WHO Essential Medicines List.

Non-typeable H. influenzae (NTHi)
ECDC/WHOH. influenzae strains lacking the polysaccharide capsule — unable to cause invasive systemic disease by the same mechanism as Hib. Instead, NTHi colonises respiratory mucosa and causes: chronic obstructive pulmonary disease (COPD) exacerbations (the most common bacterial cause); otitis media (the most common bacterial cause in children); sinusitis; community-acquired pneumonia. Invasive NTHi (bacteraemia, meningitis) is increasingly recognised in adults ≥65, immunocompromised and those with chronic lung disease.

Hib epiglottitis
WHO/PaediatricsA life-threatening acute Hib infection of the epiglottis — causing severe supraglottic oedema and airway obstruction. Classic presentation: rapid onset (hours) of high fever, severe sore throat, drooling (cannot swallow), muffled voice ("hot potato voice"), respiratory distress, sitting forward in tripod position. MANAGEMENT: do NOT examine the throat (risk of complete airway obstruction); call anaesthetics immediately; controlled intubation in theatre with surgical airway standby. Now extremely rare due to Hib vaccination.

Brazilian purpuric fever (H. influenzae biogroup aegyptius)
WHOH. influenzae biogroup aegyptius (Koch-Weeks bacillus) causes: acute purulent conjunctivitis (most common cause of epidemic conjunctivitis in children in tropical countries); and the rare but severe Brazilian purpuric fever (BPF) — a fulminant septicaemia with purpura and high mortality, first described in Brazil in 1984. BPF is caused by a specific clonal lineage of Hae.

Chemoprophylaxis for Hib contacts
WHO/ECDC/PHERifampicin prophylaxis (20mg/kg once daily × 4 days, max 600mg) is recommended for unvaccinated household contacts <4 years of age of a confirmed invasive Hib case — to eradicate nasopharyngeal carriage and prevent secondary cases. Not recommended for contacts if all children in household are fully vaccinated. Nursery/childcare contacts: consider prophylaxis if contact includes unvaccinated children under 4 years.