Pertussis is a highly contagious respiratory infection caused by the bacterium Bordetella pertussis. This pathogen attaches to the cilia lining the respiratory tract, releasing toxins that lead to airway swelling and the characteristic violent, spasmodic coughing fits. Although vaccination programs have significantly reduced its incidence, pertussis remains a global public health concern. Understanding the specific factors that increase the risk of both contracting the disease and developing severe outcomes is important for prevention.
Age and Waning Immunity
The highest risk for severe disease, hospitalization, and death is concentrated in infants under six months of age. This vulnerability stems from the fact that they are too young to have completed the primary DTaP (diphtheria, tetanus, and acellular pertussis) vaccination series, which typically begins around two months of age. Full protection is not established until after the third or fourth dose, leaving newborns largely susceptible during their most fragile period. To bridge this gap, vaccination of pregnant women between 27 and 36 weeks of gestation is recommended, allowing protective antibodies to transfer across the placenta to the fetus.
This maternally acquired immunity provides protection, reducing the risk of whooping cough in infants younger than two months by nearly 78%. However, these antibody levels naturally decline rapidly after birth, and nearly 90% of infants may lack sufficient antibody protection by six weeks of age. The primary risk shifts as children age, moving to adolescents and adults due to the phenomenon of waning immunity.
Immunity gained from childhood DTaP vaccination does not confer lifelong protection and diminishes over time. The protective effectiveness of the acellular pertussis vaccine wanes, meaning many vaccinated adolescents and adults are susceptible to infection. They often present with milder, non-classic symptoms like a persistent cough. Consequently, these individuals can unknowingly serve as a primary reservoir for the bacteria, transmitting it to highly vulnerable infants.
Exposure Settings and Transmission Risk
Pertussis spreads primarily through respiratory droplets expelled when an infected person coughs or sneezes. Transmission generally requires close, face-to-face contact, as the droplets do not remain suspended in the air for long distances. The most significant risk factor for an infant is household exposure, where the secondary attack rate is high, even among vaccinated contacts. Infected parents, older siblings, or caregivers are frequently the source of infection for infants, often before they recognize their own mild symptoms as pertussis.
High-density community environments, such as schools, college dormitories, and childcare centers, also represent settings with elevated transmission risk. These settings facilitate the spread of the airborne bacteria among groups of people, increasing the probability of exposure for all who are not fully protected. Furthermore, travel to areas with low vaccination coverage or high disease prevalence can increase the risk of exposure.
Areas with lower adherence to routine immunization schedules present a greater chance of encountering the infection. The risk is amplified in closed-setting outbreaks, where close quarters allow for easier transmission, making awareness of current community prevalence important for travelers.
Pre-existing Health Conditions and Severity
While age and vaccination status affect the risk of acquiring the infection, certain pre-existing health conditions significantly increase the risk of developing severe complications once infected. Infants born prematurely face a higher risk of severe pertussis and associated morbidity, due to their underdeveloped respiratory systems. They are more likely to experience apnea and require intensive medical intervention.
For older children and adults, chronic respiratory diseases can be worsened by the infection. Individuals with conditions like asthma or Chronic Obstructive Pulmonary Disease (COPD) are at an increased risk of severe pertussis, which can trigger acute exacerbations of their underlying lung issues. The prolonged coughing fits characteristic of the disease place immense strain on the respiratory system, leading to potentially life-threatening outcomes like pneumonia or seizures.
People with compromised immune systems, whether due to a medical condition or immunosuppressive therapy, are also considered to be at increased risk for severe disease. Although vaccination is the most effective preventative measure, the presence of these comorbidities means that vigilance and prompt diagnosis are especially important in minimizing the potential for severe outcomes, such as hospitalization.