Clinical Features

Disease caused by Haemophilus influenzae can affect many organ systems. The most common types of disease caused by H. influenzae type b (Hib) include

Less common infections include endocarditis and osteomyelitis.

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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 cause invasive disease.

Etiologic Agent

H. 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.


Between 3% to 6% of Hib cases in children are fatal. Patients ≥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.

Key Resource

Best Practices for Use of Polymerase Chain Reaction (PCR) for Diagnosing Haemophilus influenzae and Neisseria meningitidis Disease and Public Health Importance of Identifying Serotype/Serogroup



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.

Risk Groups

The following groups are at increased risk of Hib disease:

Unimmunized children younger than 5 years of ageHousehold contacts of a person with Hib diseaseDaycare classmates of a person with Hib disease

In addition, the following groups are at increased risk of H. influenzae disease:

Children younger than 5 years of ageAdults 65 years or olderAmerican Indians and Alaska NativesPeople with any of the following medical conditionsSickle cell diseaseAspleniaHIVImmunoglobulin and complement component deficienciesMalignant neoplasms requiring hematopoietic stem cell transplant, chemotherapy, or radiation therapy


The epidemiology of invasive H. influenzae disease in the United States has shifted since the introduction of the Hib vaccine. The United States began using Hib vaccine for children in 1987 and for infants in 1990. Since then, the annual incidence of invasive Hib disease in children aged younger than 5 years old decreased by 99%. Now, nontypeable H. influenzae causes the majority of invasive H. influenzae disease among all age groups in the United States. In addition:

Rates of Hib disease remain stable among adultsRates of Hib disease remain higher among Alaska Natives than for other races


In developing countries, where routine vaccination with Hib vaccine is not widely available, Hib remains a major cause of lower respiratory tract infections in infants and children.

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 immunization series.

In 2017, the incidence of invasive H. influenzae disease in children younger than 5 years old was

Hib: 0.18 cases per 100,000Non-b H. influenzae: 1.7 cases per 100,000Nontypeable H. influenzae: 1.7 cases per 100,000

Nontypeable H. influenzae now causes the majority of invasive H. influenzae disease in all age groups.

In 2017, the incidence of invasive nontypeable H. influenzae disease was 6.2 cases per 100,000 in adults 65 years of age and older.

Nontypeable H. influenzae also causes 30% to 52% of episodes of acute otitis media and sinusitis in children. It can be a common cause of recurrent otitis media.

Treatment and Chemoprophylaxis

For guidelines on treatment and chemoprophylaxis for invasive Hib disease, see the Red Bookexternal icon. 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 immunizations in the United States. Learn more:

There are no vaccines for non-b and nontypeable H. influenzae.

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Invasive H. influenzae (Hib, non-b, and nontypeable) is a reportable condition in all states. Clinicians should report all cases to through the local or state public health department.