Haemophilus influenzae type b (Hib) is a bacterium that can cause severe, invasive infections, particularly in young children. The bacterium is spread through respiratory droplets and can lead to serious conditions when it invades parts of the body that are normally sterile, such as the bloodstream or the fluid surrounding the brain. Due to the widespread use of an effective vaccine, the incidence of severe Hib disease has been reduced dramatically, making it a rare occurrence in vaccinated populations. This transformation is one of the most significant public health successes of the late 20th century, but the disease remains a threat to specific, vulnerable groups.
The Historical Burden of Hib Disease
Prior to the introduction of a widespread vaccination program, Haemophilus influenzae type b was a common and terrifying threat to children worldwide. In the United States alone, the bacterium was responsible for an estimated 20,000 cases of serious invasive disease each year before the early 1990s. The vast majority of these infections occurred in children younger than five years of age, with the highest rates seen in infants.
The annual incidence of all invasive Hib disease in the prevaccine era was high, with estimates ranging from 67 to 131 cases per 100,000 children in the United States. Hib was recognized as the leading cause of bacterial meningitis in young children. The case fatality rate for all Hib infections ranged from 3% to 7%, meaning thousands of children died annually from the disease. For those who survived, a significant percentage were left with long-term disability.
Current Incidence Rates and Surveillance
The introduction of the Hib conjugate vaccine in the late 1980s and early 1990s resulted in a precipitous decline in disease prevalence. Today, the incidence of invasive Hib disease among children under five years old has fallen by more than 99% in the United States and other developed nations. The current rate of invasive Hib disease in this age group is extremely low, reported at approximately 0.08 cases per 100,000 population.
Public health agencies track the disease through mandatory reporting systems, such as the National Notifiable Diseases Surveillance System (NNDSS) and the Active Bacterial Core surveillance (ABCs). These surveillance systems specifically monitor invasive disease, which is defined by the isolation of the Hib bacteria from a normally sterile body site, such as the blood or cerebrospinal fluid. The current data shows that fewer than 50 cases of invasive Hib disease occur annually in young children across the U.S., a stark contrast to the thousands of cases seen historically.
Surveillance focuses on invasive Hib disease, which is the most severe manifestation. Non-invasive infections caused by Haemophilus influenzae, such as ear infections or bronchitis, are not routinely tracked by these systems and are often caused by non-type b strains. While the overall number of H. influenzae infections has increased due to non-type b and non-typeable strains, the incidence of the vaccine-preventable type b disease remains successfully suppressed.
Populations Still at Risk
Despite the success of vaccination, Hib disease persists in specific populations who lack protection. Unvaccinated or undervaccinated children remain the primary group susceptible to infection. For instance, a review of cases in young children found that a significant portion occurred in those who had not received any or all of the recommended vaccine doses.
Infants who are too young to have completed the full vaccine series are also highly vulnerable, as the highest rates of invasive infection occur in children under one year old. Although rare, vaccine failure can occur, where an age-appropriately vaccinated child still contracts the disease. This highlights that no vaccine offers 100% protection, though the Hib vaccine is remarkably effective.
Individuals with certain underlying medical conditions face a higher risk of contracting Hib disease, even if vaccinated. These conditions compromise the immune system’s ability to fight off the bacteria. Specific risk factors include:
- Asplenia (having no spleen)
- Sickle cell disease
- HIV infection
- Immunoglobulin or complement deficiency syndromes
- Patients undergoing chemotherapy, radiation therapy, or bone marrow stem cell transplants for cancer
Severe Clinical Manifestations
The reason Hib prevention is so rigorously pursued is the life-threatening nature of the resulting infections. Invasive Hib disease can manifest as several severe clinical syndromes, including bacterial meningitis, which is an infection of the membranes covering the brain and spinal cord. Meningitis is the most feared complication, often presenting with fever, a stiff neck, and altered mental status.
Another severe presentation is epiglottitis, where the flap of tissue covering the windpipe swells rapidly, potentially causing fatal airway obstruction. Hib can also cause bacteremia, which is a dangerous bloodstream infection that can spread the bacteria throughout the body. Other invasive forms include severe pneumonia, septic arthritis, and cellulitis. Even with modern antibiotic treatment, up to 20% of children who survive the infection are left with permanent neurological sequelae, such as hearing loss or intellectual disability.