Transmission is by inhalation of respiratory tract droplets or by direct contact with respiratory tract secretions. Before the introduction of the vaccine, H. inﬂuenzae type b (Hib) caused 95 percent of H. inﬂuenzae invasive disease in infants and children. Hib causes meningitis and other focal infections (such as pneumonia, septic arthritis and cellulitis) in children, primarily those aged under 2 years, while epiglottitis was more common in children over 2 years. Invasive Hib disease was rare over the age of 5 years, but could occur in adults. The incubation period of the disease is unknown, but is probably from two to four days.
Prior to immunisation, the most common presentations of Hib invasive disease in New Zealand were meningitis and epiglottitis. Meningitis tends to occur in younger children aged between 3 months and 3 years, while epiglottitis usually occurs in children aged between 2 and 4 years. In the absence of vaccination these presentations may still occur. There have always been a small number of cases of H. inﬂuenzae invasive disease in adults, and these continue to occur.
Non-typeable H. inﬂuenzae (NTHi) organisms usually cause non-invasive mucosal infections, such as otitis media, sinusitis and bronchitis, but can occasionally cause bloodstream infection, especially in neonates. They are frequently present (60–90 percent) in the normal upper respiratory tract ﬂora. Immunisation against Hib does not protect against infections due to other H. inﬂuenzae types or NTHi strains.
Young infants (aged under 2 years) do not produce an antibody response following Hib invasive disease, so a course of Hib vaccine is recommended when they have recovered (see section 6.5.3).
H. inﬂuenzae type b and NTHi strains also cause diseases (including pneumonia and septicaemia) in the elderly.