Haemophilus influenzae serotype A
Haemophilus influenzae type a (Hia) is the second most virulent serotype after serotype b. The first cases of Hia were first described in the early 1930s in the USA and in 2017 Hia cases have been reported in approximately 35 countries including Canada, Papa New Guinea and South Africa and therefore in 6 out of 7 continents.
Risk population
As with Hib disease, children below 5 years, adults with underlying health conditions and indigenous populations of North America are the most vulnerable populations to Hia infections. The case fatality rate due is highly variable, ranging from 5.6% to 33% and is dependent on the patient’s age and Hia strain.
Disease Epidemiology
Hia can cause disease of similar presentation and severity to Hib including meningitis, pneumonia, septic arthritis and bacteraemia. It is responsible for low rates of endemicity, outbreaks and recurrent infections of meningitis and bacteremia. Since the early 2000s, several cases and outbreaks of Hia infections have been documented in the North American region (Alaska, Nunavik and Nunavut) (Tsang et al 2017).
Hia infections have been documented in several other countries but it is unclear whether the apparent increase in Hia cases is due to replacement of Hib, improved monitoring and reporting of Hi disease or the identification of previously unidentified cases resulting from enhanced serotyping practices.
Vaccines
Presently there are no vaccines licensed for use against Hia and the available Hib vaccines do not provide cross-protection against Hia. Proposals for the development of Hia vaccines were made in 2007 and provision thereof can never be overemphasised as the clinical presentation of the disease in children is non-specific.
