Haemophilus influenzae type b

What is Haemophilus influenzae type b (Hib)?

Hib is an encapsulated strain of bacteria that colonizes the human nose and throat. The bacteria may spread throughout the body through the bloodstream and may become life-threatening.1

A polysaccharide, polyribosylribitol phosphate, or PRP, composes the outer layer of the Hib bacterium and is an important determinant in the disease virulence.1

What invasive diseases are caused by Hib?

Common Types of Invasive Diseases Caused by Hib Prior to 1990.1

Chart of Diseases Caused By Invasive Haemophilus Influenzae type b (Hib) Prior to 1990
Chart of Diseases Caused By Invasive Haemophilus Influenzae type b (Hib) Prior to 1990

Symptoms and Complications of Diseases Caused by Hib

Meningitis: 50% Meningitis occurs when the bacterium invades the meninges (three protective layers of the brain membrane that contain fluids and tissue) and causes inflammation and swelling. Classic symptoms include: fever, bulging fontanelle (soft spot on top of baby’s head), stiff neck, and decreased mental state. In young infants, symptoms may also include altered alertness, poor feeding, and irritability. Long-term complications from meningitis could include: mental retardation, cerebral palsy, hearing loss, and seizure disorders.1,2
Epiglottitis: 17% Epiglottitis occurs when the bacterium invades the epiglottis (flap in the throat that covers the windpipe) and surrounding tissue and causes inflammation and swelling. Symptoms include: fever, severe sore throat, abnormal and high-pitched sound when breathing, difficult and painful swallowing, drooling, anxious and restless behavior, and discomfort when sitting up or leaning forward. Swelling may lead to airway obstruction and death within hours if left untreated.2,3
Pneumonia: 15% Pneumonia caused by Hib is common in children (4 months to 4 years old) and is difficult to differentiate from other bacterial pneumonias. It typically presents in winter and spring, and severe cases may require hospitalization.3
Arthritis: 8% Arthritis refers to septic arthritis, which is when the infection occurs in the fluids in the joints.1,2 Symptoms include: joint pain, swelling, redness of the skin, limited range of motion, and fever.2
Cellulitis: 6% Cellulitis can be characterized as a fast-progressing skin infection involving the neck, head, and face.1
Osteomyelitis: 2% Osteomyelitis refers to infection in the bone.1
Bacteremia: 2% Bacteremia refers to infection in the blood without the presence of infection within a specific organ. Symptoms may include: fever, poor feeding, and lack of energy.3
How is Hib transmitted?

Hib bacteria are thought to spread primarily through contact with respiratory droplets expelled from an infected person. Although the contagious potential of the Hib bacteria is thought to be limited, close contact with case-patients (eg, childcare, household, or institutional setting) can also lead to disease transmission.1

What is the pathology of Hib?

The bacteria enter through the nasopharynx, where they begin to colonize. The bacteria may remain there for several months without symptoms. In some people, the bacteria cause invasive Hib disease.1

Although uncommon, a person can become ill up to 60 days following contact with a child with Hib disease. This is referred to as secondary transmission, which accounts for <5% of all Hib disease.1

How do you diagnose Hib disease?

Confirmation of Hib disease may require various blood or fluid samples (eg, spinal fluid and other body cavity fluids) to be cultured on appropriate media. A positive culture for H. influenzae establishes the diagnosis.1,3

What are the risk factors for invasive Hib disease?

Individual Factors Contributing to Higher Risk1,2

Individual Risk Factors for Haemophilus Influenzae type b (Hib) Disease Ethnicity – among African Americans, Hispanics, Native Americans
Gender – in males
Age – in those <2 years old
Other illnesses – especially those that affect the immune system

Population Factors Contributing to Higher Risk1,2

  • Large, crowded households
  • Families with school-aged siblings
  • Daycare or school attendance
  • Low socioeconomic status
  • Low parental education level
  • Large, crowded households
  • Families with school-aged siblings
  • Daycare or school attendance
  • Low socioeconomic status
  • Low parental education level
Why is invasive Hib disease more prevalent in children?

The human body can usually fight against Hib infection by developing antibodies called anti-PRP (antibody against the bacterium’s outer layer composed of the polysaccharide, polyribosylribitol phosphate [PRP]).3

Children Up to 6 Years Old Were Most Susceptible to Invasive Hib Disease (prevaccine era)1,3

Anti-PRP levels

Children Up to 6 Years Old Are Most Susceptible to Haemophilus Influenzae type b (Hib) Disease Children Up to 6 Years Old Are Most Susceptible to Haemophilus Influenzae type b (Hib) Disease
Risk Period of Symptomatic Invasive Disease

0-24 months

3 years old

4 years old

5 years old

6 years old

0-24 months

Anti-PRP transferred from the mother during pregnancy or breastfeeding begins to decline at birth and reaches the lowest level at 6 months. In the prevaccine era, up to 60% of invasive disease occurred before age 12 months, with a peak occurrence among children 6–11 months of age.1

3 to 5 years old

At this age, children gradually acquire anti-PRP through repeated exposure and therefore are more susceptible to disease.

6 years old

Over time, children naturally acquire anti-PRP, and invasive disease is rare in children aged 6 years and older.

Usually, children over 6 years old and adults have naturally acquired enough anti-PRP that they are no longer susceptible to invasive Hib disease.3

CDC Disease Information

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CDC=Centers for Disease Control and Prevention; Hib=Haemophilus influenzae type b.



  1. Centers for Disease Control and Prevention (CDC). Haemophilus influenzae type b. In: Hamborsky J, Kroger A, Wolfe S, eds. Epidemiology and Prevention of Vaccine-Preventable Diseases. 13th ed. Washington, DC: Public Health Foundation; 2015:119–134. http://www.cdc.gov/vaccines/pubs/pinkbook/index.html. Accessed December 9, 2019.


  1. Nanduri SA, Sutherland AR, Gordon LK, et al. Haemophilus Influenzae Type B Vaccines. In: Plotkin SA, Orenstein WA, Offit PA, Edwards KM, eds. Plotkin’s Vaccines. 7th ed. Philadelphia, PA: Elsevier; 2018: 301-318.


  1. Murphy TF. Haemophilus Species, Including H. influenzae and H. ducreyi (Chancroid). In: Bennett JE, Dolin R, Blaser MJ, eds. Mandell, Douglas and Bennett’s Principles and Practice of Infectious Diseases. Vol 2. 9th ed. Philadelphia, PA: Elsevier; 2019:2743-2752.

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