Haemophilus influenzae are gram-negative bacteria that occur as encapsulated typeable forms (serotypes a through f, such as Hib) or as unencapsulated nontypeable strains [1]. They are capable of causing disease across age groups and may be carried in the nasopharynx without symptoms [1].
Disease overview
Bacterial Evidence-backed information EN 6/7 applicable sections · ZH 6/7 applicable sections Updated Oct 23, 2026
Haemophilus influenzae are gram-negative bacteria that may be encapsulated (typeable serotypes a–f) or unencapsulated (nontypeable) and can affect multiple organ systems [1]. Globally, H. influenzae accounted for an estimated 22% of reported bacterial meningitis episodes in a large multicountry review, reflecting its continued contribution to invasive disease [2]. Preventive efforts include Hib chemoprophylaxis for specified close contacts and routine use of conjugate vaccines in immunization schedules [1][3].
Read the full clinical and epidemiological profile 7
Typeable H. influenzae including Hib most commonly produce invasive syndromes, while less frequent manifestations include endocarditis and osteomyelitis [1]. Nontypeable strains frequently cause acute otitis media and sinusitis in children and bronchitis in adults, and can also cause invasive disease in people of all ages [1].
In a global systematic review of community-acquired bacterial meningitis from 1935 to 2019, H. influenzae was identified as the causative pathogen in 22,722 of 104,598 episodes with known etiology (22%) [2]. The overall case fatality of bacterial meningitis was 18%, though etiology-specific fatality for H. influenzae was not separately reported [2]. In 2019, global meningitis modeling estimated H. influenzae among the recognized aetiologies contributing to 2.51 million incident cases and 236,000 deaths, with the highest burden in children under five years [4].
H. influenzae spreads when a person inhales or has direct contact with respiratory droplets from symptomatic individuals or nasopharyngeal carriers who cough or sneeze [1]. Newborns may acquire infection through aspiration of infected amniotic fluid or contact with genital-tract secretions during delivery [1].
CDC recommends chemoprophylaxis with Hib-specific regimens for specified close contacts of cases and directs clinicians to current prevention guidance [1]. Immunization schedules incorporate Haemophilus influenzae type b conjugate vaccines for infants and children, with updated guidance expanding routine pediatric and adolescent vaccine use [3].
GlobalID source-data note: Ontario, Ireland, Iceland, Norway, and Sweden report invasive or systemic Haemophilus influenzae disease. Finland also publishes a total and separate serotype components. These are source-defined surveillance slices: do not interpret invasive counts as all H. influenzae infections, and do not add a reported total to its serotype components.
- 1 US Centers for Disease Control and Prevention. Clinical Overview of Haemophilus influenzae Disease | H. influenzae | CDC [Internet]. cited 23 Oct 2026. Available from: https://www.cdc.gov/hi-disease/hcp/clinicians/index.html
- 2 van Ettekoven CN et al. Global Case Fatality of Bacterial Meningitis During an 80-Year Period: A Systematic Review and Meta-Analysis. JAMA Netw Open. 2024 Aug 1. PMID: 39093565. doi: 10.1001/jamanetworkopen.2024.24802. PubMed: https://pubmed.ncbi.nlm.nih.gov/39093565/
- 3 General recommendations on immunization. Recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR Recomm Rep. 1994 Jan 28. PMID: 8145710. PubMed: https://pubmed.ncbi.nlm.nih.gov/8145710/
- 4 GBD 2019 Meningitis Antimicrobial Resistance Collaborators et al. Global, regional, and national burden of meningitis and its aetiologies, 1990-2019: a systematic analysis for the Global Burden of Disease Study 2019. Lancet Neurol. 2023 Aug. PMID: 37479374. doi: 10.1016/S1474-4422(23)00195-3. PubMed: https://pubmed.ncbi.nlm.nih.gov/37479374/
- 5 Pickering LK et al. Immunization programs for infants, children, adolescents, and adults: clinical practice guidelines by the Infectious Diseases Society of America. Clin Infect Dis. 2009 Sep 15. PMID: 19659433. doi: 10.1086/605430. PubMed: https://pubmed.ncbi.nlm.nih.gov/19659433/
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