Haemophilus influenzae is a bacterium that lives in the nose and throat and can cause infections ranging from mild ear infections to life-threatening meningitis and bloodstream infections. Despite its name, it has nothing to do with influenza (the flu). The bacterium was discovered during the 1892 flu pandemic and mistakenly thought to cause the disease, and the misleading name stuck.
Encapsulated vs. Non-Typeable Strains
Not all Haemophilus influenzae bacteria are the same. The species splits into two broad categories based on whether the bacterium has a protective sugar coating called a polysaccharide capsule.
Encapsulated strains are classified into six serotypes, labeled a through f, based on the chemical makeup of that capsule. The capsule acts like armor, helping the bacterium evade the immune system and making these strains more likely to cause severe, invasive infections. Type b (known as Hib) was historically the most dangerous, especially for young children.
Non-typeable strains (often abbreviated NTHi) lack this capsule entirely. They use a different playbook to survive: instead of hiding behind a sugar shell, they attach directly to the cells lining the airways and can burrow into the tissue beneath. They also form sticky, hard-to-clear colonies called biofilms, which help them resist both the immune system and antibiotics. These strains are the primary culprits behind ear infections, sinusitis, bronchitis, and flare-ups of chronic lung disease in adults.
How It Spreads
Haemophilus influenzae travels through respiratory droplets. When someone coughs or sneezes, the bacteria become airborne in tiny droplets that others can inhale. Close or prolonged contact with an infected person also allows transmission.
The key detail most people don’t realize: the bacteria spread most often from people who carry them without any symptoms at all. Someone can harbor H. influenzae in their nose and throat, feel perfectly healthy, and still pass the bacterium to others.
Diseases It Causes
The infections caused by H. influenzae fall into two categories. Non-invasive infections stay in the airways and include ear infections, sinusitis, and bronchitis. These are common, usually caused by non-typeable strains, and typically treatable without hospitalization.
Invasive infections occur when the bacterium enters parts of the body that are normally sterile, like the blood, the fluid surrounding the brain, or the lining of the joints. These are medical emergencies. The most serious invasive infections include:
- Meningitis: infection of the membranes surrounding the brain and spinal cord. Symptoms come on suddenly and include fever, severe headache, stiff neck, nausea, sensitivity to light, and confusion. In babies, signs are less specific: irritability, poor feeding, vomiting, and unusual sluggishness.
- Bloodstream infection (bacteremia): symptoms include fever, chills, extreme fatigue, abdominal pain, shortness of breath, and confusion. A bloodstream infection can occur on its own or alongside pneumonia or meningitis.
- Epiglottitis: a rapidly progressing swelling of the tissue at the base of the tongue that can block the airway, making it difficult or impossible to breathe.
The stakes with invasive disease are high. Between 3% and 6% of Hib cases in children are fatal. Adults over 65 face even higher fatality rates. Among children who survive Hib meningitis, up to 20% are left with permanent hearing loss or other long-term neurological problems. Severe bloodstream infections can, in rare cases, lead to limb loss.
Its Role in Chronic Lung Disease
Non-typeable H. influenzae plays an outsized role in chronic obstructive pulmonary disease (COPD). Multiple large studies have identified NTHi as the single most common bacterium found in the airways of people with COPD, detected in the sputum of roughly 70% of patients with moderate to severe disease in one cohort of 105 patients.
During acute flare-ups of COPD, NTHi populations surge compared to baseline levels. Higher bacterial loads correlate with more severe airway inflammation, more frequent hospitalizations, and greater overall disease burden. In patients with very severe COPD, NTHi is significantly more prevalent than in those with moderate disease. One long-term study following 181 COPD patients over four and a half years found NTHi was the most commonly isolated bacterium across nearly 9,000 clinic visits, present in about 14% of cultures.
Antibiotic Resistance Patterns
H. influenzae is often treated with ampicillin-type antibiotics, but resistance is a growing concern. A global analysis of nearly 14,000 isolates collected between 2013 and 2022 found that about 72% remained susceptible to ampicillin, meaning roughly 28% showed some form of resistance.
One particularly worrisome trend involves strains that resist ampicillin through a mechanism that standard rapid tests don’t always catch. These strains, called BLNAR (beta-lactamase-negative ampicillin-resistant), appear at strikingly different rates around the world. In Europe and Latin America, fewer than 1% of isolates fall into this category. In North America, the rate is about 4%. But in parts of Asia the picture is far worse: Vietnam (87.5%), Japan (70.8%), South Korea (65.7%), and Taiwan (65.9%) show the highest BLNAR rates globally. This geographic variation matters because it affects which antibiotics doctors choose for treatment.
How the Hib Vaccine Changed Everything
Before a vaccine existed, Hib was the leading cause of bacterial meningitis in children under five in the United States. The introduction of Hib conjugate vaccines in the late 1980s changed the landscape dramatically. Invasive Hib disease has declined by more than 99% since the pre-vaccine era.
The current vaccine schedule calls for a primary series starting as early as six weeks of age, with doses given at 2, 4, and 6 months (or two doses at 2 and 4 months depending on the vaccine brand), followed by a booster between 12 and 15 months. The booster needs to be given at least eight weeks after the most recent dose.
The Hib vaccine is also recommended for older children and adults in specific situations, including those who have undergone a stem cell transplant, those without a functioning spleen (including people with sickle cell disease), and those with certain immune deficiencies like HIV or complement deficiency.
One important limitation: the Hib vaccine protects only against serotype b. It does nothing against the other five encapsulated serotypes or the non-typeable strains responsible for ear infections, sinusitis, and COPD flare-ups. There is currently no licensed vaccine against non-typeable H. influenzae, which is why these strains remain a persistent problem, particularly in adults with chronic lung conditions.