Haemophilus influenzae

Haemophilus influenzae infections have high mortality rates, and often result in severe long-term complications even in those who are successfully treated. The most prevalent subtype, which causes most cases of severe disease, is H. influenzae type b (Hib).

Mechanism of transmission: H. influenzae is spread in respiratory droplets, by sneezing and coughing.

Number of cases: the World Health Organisation estimates 3 million cases of serious disease occur worldwide each year, with up to 400,000 deaths^{1, 2}. The majority of these cases occur in unimmunised children and in low-income countries.

Haemophilus influenzae type b (Hib)

Haemophilus influenzae bacteria can be categorised into encapsulated and non-encapsulated, based on the presence or absence of an outer protective coating known as a capsule. The presence of a capsule makes the bacteria more virulent (more infectious and causing more severe disease). Encapsulated H. influenzae are further divided into serotypes (subtypes) ‘a’ to ‘f’.

Haemophilus influenzae type b (Hib) is the most common serotype and is responsible for 95% of severe infections in unimmunised populations. Hib infections range from mild (such as mild ear infections or bronchitis) to severe invasive disease. Babies and young children are most at risk of serious disease, along with the elderly and those with a weakened immune system (for example due to medical treatments such as chemotherapy, or conditions such as HIV or sickle cell disease).

Invasive diseases (infection of parts of the body that are normally free from bacteria) that can be caused by Hib include:

Pneumonia – infection of the lungs, and blood and fluid surrounding the lungs.
Meningitis – infection of the membranes surrounding the brain and spinal cord.
Epiglottitis – infection of the epiglottis, the ‘lid’ that covers the windpipe at the back of the throat.
Septic arthritis – infection of the joints.
Septicaemia – extreme overreactive immune response to an infection, resulting in tissue damage, organ failure, and death if not treated.

Hib infections can be life-threatening. Mortality rates can reach 5–10% in high-income countries, and up to 40% in low-income countries, even with antibiotic treatment¹. Even with successful treatment, infections can lead to severe long-lasting side effects such as loss of limbs, blindness, epilepsy and seizures, and behavioural and learning disabilities in children.

Note that, despite the name, Haemophilus influenzae does not cause influenza (flu).

Vaccination

Immunisation (vaccination) is the only public health intervention capable of preventing the majority of serious Haemophilus influenzae infections. An effective and safe vaccination has been available against Hib since the 1980s, and is recommended by the World Health Organisation for use as part of routine vaccination programmes.

The Hib vaccine contains a polysaccharide (a type of carbohydrate molecule) that is found on the surface of Hib bacteria, known as polyribosylribitol phosphate (PRP). The vaccine trains the immune system to recognise PRP, so if a H. influenzae cell invades it can be quickly identified and destroyed.

In the UK, Hib is vaccinated against as part of the NHS routine vaccination schedule, as part of two different vaccines:

The 6-in-1 vaccine – three doses are given to babies at 6, 12, and 18 weeks; this vaccine protects against six serious illnesses (Hib, along with diphtheria, polio, tetanus, hepatitis B, and whooping cough).
The Hib/MenC vaccine – a single dose is given to babies at 1 year old; this vaccine boosts protection against Hib and also protects against another serious disease, meningitis C.

Genomics and Haemophilus influenzae

Haemophilus influenzae can cause severe diseases with high mortality rates and resulting in serious complications even when treated successfully. Genomic data can be used to help increase our understanding of H. influenzae, which is essential for tackling these diseases. For example, genome sequencing can help to identify outbreak serotypes, monitor trends in case numbers across the globe, and monitor the impact of vaccination programs.

If you are keen to find out more about Haemophilus influenzae, these websites are good places to go for more information: