Pertussis, commonly known as whooping cough, is a highly contagious respiratory illness caused by the bacterium Bordetella pertussis. It spreads easily through the air when an infected person coughs or sneezes. While childhood vaccination programs have drastically reduced the incidence of this disease, they have not eliminated it entirely. Breakthrough cases, where a vaccinated individual still contracts the illness, are possible. Understanding why infections can still occur is necessary for public health strategy and personal protection.
How the Pertussis Vaccine Works
The pertussis vaccine used today is an acellular vaccine, meaning it contains only purified, inactivated components of the Bordetella pertussis bacteria, rather than the entire dead cell. These components are specific antigens, such as pertussis toxin and filamentous hemagglutinin, which the body’s immune system learns to recognize. The pediatric version is called DTaP, administered in a series of five doses to children, while the booster for adolescents and adults is called Tdap.
The vaccine works by stimulating the immune system to produce protective antibodies against the pertussis antigens. Shortly after completing the primary DTaP series, the vaccine demonstrates high efficacy, protecting up to 98% of children in the year following the last dose. This initial robust protection is crucial for safeguarding infants, who are at the highest risk for severe complications. The primary goal of the vaccine is to prevent severe disease and death, which it achieves even if it does not entirely block bacterial colonization.
Why Infections Still Occur
The primary reason for breakthrough infections is the gradual decline of vaccine-induced protection over time, known as waning immunity. Unlike immunity from natural infection or the older whole-cell vaccine, the protection offered by the current acellular vaccine is temporary. Efficacy begins to drop noticeably within a few years after the last dose, especially following the final childhood DTaP booster.
The odds of contracting pertussis increase by approximately 33% with every year that passes after the fifth DTaP dose. For instance, the protection rate drops to about 71% five years after that final dose. This decline is why booster shots, particularly the Tdap vaccine for adolescents and adults, are recommended to restore protective antibody levels.
A secondary factor contributing to recurring infections involves evolutionary changes in the Bordetella pertussis bacteria itself. Some circulating strains have shown antigenic divergence from the strains used to formulate the vaccine. These changes may allow the bacteria to evade the immune response more easily, particularly in individuals whose vaccine-induced immunity is already low.
Difference in Disease Severity
The most significant benefit of vaccination is its ability to reduce the severity and duration of the illness, even in breakthrough cases. Vaccinated individuals who contract pertussis typically experience a milder clinical course compared to those who are unvaccinated. Studies have shown that vaccinated patients are five times less likely to be hospitalized and about two-and-a-half times less likely to develop severe illness.
For those who are up-to-date on their vaccinations, the disease presentation is less severe, often resembling a prolonged cough rather than the characteristic violent, paroxysmal coughing fits. The duration of the spasmodic cough is also decreased, sometimes by eight to ten days compared to unvaccinated individuals. Complications such as seizures, acute encephalopathy, and pneumonia are significantly less common among vaccinated patients.
The protective effect is particularly important for infants. When pregnant individuals receive the Tdap vaccine, the antibodies are passed to the fetus, offering protection to the newborn before they are old enough to receive their first DTaP dose. This maternal vaccination prevents about 78% of pertussis cases and 90% of related hospitalizations in infants younger than two months old.
Steps to Take After Exposure or Symptoms
If you suspect you have been exposed to pertussis or begin to develop symptoms, contact your healthcare provider immediately to discuss your symptoms and exposure history. Pertussis should be suspected if you have a prolonged cough, especially one that includes violent, rapid coughing fits or is followed by vomiting.
Diagnosis and Treatment
Diagnostic testing typically involves a nasopharyngeal swab or aspirate, which is sent for polymerase chain reaction (PCR) testing to detect the bacterial DNA. If confirmed, or if clinical suspicion is high, antibiotic treatment is recommended, usually a macrolide like azithromycin or clarithromycin. Antibiotics are most effective at reducing the severity and contagiousness if started within the first three weeks of the cough onset.
Post-Exposure Prophylaxis (PEP)
For close contacts, especially those in high-risk categories such as infants, pregnant individuals, or those with underlying health conditions, post-exposure prophylaxis (PEP) with antibiotics may be recommended. PEP aims to prevent the infection entirely and should be administered within 21 days of the last known exposure. Even if you are fully vaccinated, consulting a doctor after exposure is important to protect yourself and those around you.