What is Haemophilus Influenza Type B (Hib)?

Haemophilus influenzae (H. influenzae) is a small gram-negative coccobacillus⁽¹⁾ present only in humans⁽²⁾ and can be encapsulated (typeable) or unencapsulated (non-typeable). Current vaccines against H. influenzae affect only H. influenzae type b, which was thought to be the cause of 95% of all H. influenzae infections in the pre-vaccine era.⁽³⁾

The H. influenzae type b (Hib) is transmitted by direct contact with respiratory secretions and enters the body through the upper respiratory tract. 

The diagnosis of H. influenzae, including H. influenzae type b is done through laboratory analysis of bodily fluids such as blood and spinal fluid.⁽⁴⁾

While invasive disease caused by H. influenzae type b (Hib) has decreased, invasive disease caused by non-b, nontypable H. influenzae has increased.^{(5-6-7-8-9-10-11-12-13-14)} Non-b type H. influenzae infections, such as H. influenzae type a, can cause invasive disease with complications similar to those of Hib⁽¹⁵⁾ and nontypable invasive H. influenzae disease, although it is most commonly associated with infections such as conjunctivitis, otitis media, sinusitis, and pneumonia, still has the potential to cause severe invasive H. influenzae disease.⁽¹⁶⁾ Hib vaccines only target H. influenzae type b offer no protection against any other type of H. influenzae infection.⁽¹⁷⁾

References:

1. CDC Haemophilus influenzae type b – Haemophilus influenzae Epidemiology and Prevention of Vaccine-Preventable Diseases (The Pink Book). 13th ed. 2015.
2. CDC Haemophilus influenzae type b – Epidemiology Epidemiology and Prevention of Vaccine-Preventable Diseases (The Pink Book). 13th ed. 2015.
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4. CDC Haemophilus influenzae Disease (including Hib) Diagnosis, Treatment, and Complications. Feb. 13, 2018
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13. Langere JD, de Jonge MI Invasive Disease Caused by Nontypeable Haemophilus influenzae Emerg Infect Dis. 2015 Oct; 21 (10): 1711–1718.
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17. 32 CDC Haemophilus influenzae Disease (including Hib) – Prevention. Feb. 13, 2018