Haemophilus influenzae is a pleomorphic gram-negative coccobacillus. H. influenzae may be either encapsulated (typeable) or unencapsulated (nontypeable). There are 6 encapsulated serotypes (designated a through f) that have distinct capsular polysaccharides.
Disease caused by H. influenzae can affect many organ systems. The most common types of disease caused by H. influenzae type b (Hib) include
- Septic arthritis
- Otitis media
- Purulent pericarditis
Less common infections include endocarditis and osteomyelitis.
Non-b but typeable (i.e., a, c, d, e, and f) H. influenzae (hereto after referred to as non-b H. influenzae) can cause disease similar to Hib infections. Nontypeable H. influenzae commonly causes ear infections in children and bronchitis in adults, but can also cause invasive disease.
Between 3% to 6% of Hib cases in children are fatal. People ≥65 years of age with invasive H. influenzae disease (Hib, non-b, and nontypeable) have higher case-fatality ratios than children. Up to 20% of patients who survive Hib meningitis have permanent hearing loss or other long-term neurological sequelae.
Transmission occurs through direct contact with respiratory droplets from a nasopharyngeal carrier or case patient. Neonates can acquire infection by aspiration of amniotic fluid or contact with genital tract secretions containing the bacteria.
The following groups are at increased risk of Hib disease:
- Unimmunized children younger than 5 years of age
- Household contacts of a person with Hib disease
- Daycare classmates of a person with Hib disease
In addition, the following groups are at increased risk of H. influenzae disease (caused any serotype or nontypeable bacteria):
- Children younger than 5 years of age
- Adults 65 years or older
- American Indian and Alaska Native people
- People with any of the following medical conditions
- Sickle cell disease
- Immunoglobulin and complement component deficiencies
- Malignant neoplasms requiring hematopoietic stem cell transplant, chemotherapy, or radiation therapy
See the H. influenzae surveillance page for information on disease trends since 1999.
In the United States, Hib disease is not common. It occurs primarily in underimmunized children and in infants too young to have completed the primary vaccination series. Nontypeable H. influenzae now cause the majority of invasive H. influenzae disease in all age groups. Nontypeable H. influenzae also cause 30% to 52% of episodes of acute otitis media and sinusitis in children. It can be a common cause of recurrent otitis media.
All age groups
In 2019, the incidence of invasive H. influenzae disease was
- Hib: 0.04 cases per 100,000
- Non-b H. influenzae: 0.52 cases per 100,000
- Nontypeable H. influenzae: 1.34 cases per 100,000
Children younger than 5 years old
In 2019, the incidence of invasive H. influenzae disease in children younger than 5 years old was
- Hib: 0.15 cases per 100,000
- Non-b H. influenzae: 1.18 cases per 100,000
- Nontypeable H. influenzae: 1.62 cases per 100,000
Adults 65 years of age or older
In 2019, the incidence of invasive H. influenzae disease in adults 65 years of age and older was
- Hib: 0.06 cases per 100,000
- Non-b H. influenzae: 1.33 cases per 100,000
- Nontypeable H. influenzae: 4.88 cases per 100,000
See the H. influenzae surveillance page for information on disease incidence and trends.
For guidelines on treatment and chemoprophylaxis for invasive Hib disease, see the H. influenzae chapter of the Red Bookexternal icon. CDC recommends chemoprophylaxis for close contacts of Hib cases, but does not have guidelines for other types of H. influenzae disease.
Hib vaccine is one of the recommended routine childhood vaccinations in the United States. See Hib vaccination: Information for healthcare professionals for information on all Hib vaccine recommendations by age and indication.
There are no vaccines for non-b and nontypeable H. influenzae.
Invasive H. influenzae (Hib, non-b, and nontypeable) is a reportable condition in all states. Clinicians should report all cases to CDC through the local or state public health department.
- Briere EC. Food and Drug Administration approval for use of Hiberix as a 3-dose primary Haemophilus influenzae type b (Hib) vaccination series. MMWR Morb Mortal Wkly Rep. 2016;65(16):418–419.
- Briere EC, Rubin L, Moro PL, Cohn A, Clark T, Messonnier N. Prevention and control of Haemophilus influenzae type b disease: Recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR Morb Mortal Wkly Rep. 2014;63(RR01):1–14.
- Blain A, MacNeil, J, Wang X, et al. Invasive Haemophilus influenzae disease in adults ≥65 years, United States, 2011external icon. Open Forum Infect Dis. 2014;1(2): ofu044.
- MacNeil JR, Cohn AC, Farley M, et al. Current epidemiology and trends in invasive Haemophilus influenzae disease—United States, 1989–2008external icon. Clin Infect Dis. 2011;53:1230–6.
- Rubach MP, Bender JM, Mottice S, et al. Increasing incidence of invasive Haemophilus influenzae disease in adults, Utah, USA. Emerg Infect Dis. 2011;17:1645–50.