H. influenzae is a gram-negative bacterium that can be either encapsulated (6 serotypes have been identified, from a to f) or non-encapsulated (known as “non-typeable”). 
Of the different H. influenzae, H. influenzae type b (Hib) is the most pathogenic in humans. In Belgium, before the introduction of the vaccine (against only Hib) in 1993, it was the most common cause of bacterial meningitis in children under 5 years of age. It can also cause pneumonia, epiglottitis, arthritis, cellulitis and septicaemia.

Most invasive infections currently occur in elderly patients with underlying medical conditions and are caused mainly by non-typeable strains followed by serotypes other than type b.

Transmission ofH. influenzae is person-to-person, by inhalation of respiratory droplets (generated by sneezing, coughing or talking to an infected person or asymptomatic carrier) or by contact with objects contaminated with respiratory tract secretions. The bacteria may remain in the mucous membranes of the throat and nose without causing symptoms (asymptomatic carriage). 

Invasive Haemophilus influenzae can be confirmed by isolation of the bacteria or detection of nucleic acid from a normally sterile anatomical site (such as cerebrospinal fluid, blood, joint fluid, pleural effusion, etc.). The confirmation of type b is performed by the National Reference Centre (NRC). It is therefore important to send the strain to the NRC in case of a positive culture for Haemophilus influenzae and an invasive infection clinic. 

The treatment comprises the prompt use of appropriate antibiotics.. 

A vaccine exists only for H. influenzae type b. In Belgium, the Hib vaccination was recommended by the Superior Health Council in 1993. It was made free in 2002 as part of the community vaccination programme (four doses at 2, 3, 4 and 15 months). You can find the Superior Health Council’s advice regarding vaccination against Hib and the basic vaccination schedule in Wallonia — Brussels Federation and in the Flemish Community. 

Before the Hib vaccination, this serotype was the main cause of invasive infections. 

Age is an important risk factor for the occurrence of invasive Hib infections. Children under 5 years of age and, particularly those under 1 year of age, who are not vaccinated or not fully vaccinated are particularly at risk. Children aged 2-10 years and adults are at higher risk for epiglottitis (if not vaccinated or not fully vaccinated).

Since the introduction of the Hib vaccination, it is, rather, children under one year of age and people over 65 years of age who are more likely to be affected, mainly by unencapsulated strains followed by non-b serotypes. Currently, most invasive infections occur in elderly patients with underlying conditions such as immunocompromised individuals or those with respiratory disease.