During rounds in spring 2012, my fellow’s case presentation was interrupted by a patient’s prolonged coughing episode. Asthma? The fellow paused, but as soon as she resumed her presentation, the patient started coughing again, unrelenting. Pertussis? We walked past the patient’s shared room—no respiratory precautions sign. Like us, the primary team’s initial impression had also been asthma. But the patient was indeed found to be pertussis positive, after she had exposed multiple healthcare workers and patients on a busy general pediatrics floor.
In 2012, there were more than 48 000 cases of pertussis in the United States, the highest number of cases since vaccine introduction in the 1940s, according to the Centers for Disease Control and Prevention. Incidence had been increasing for several years and has remained high, with nearly 29 000 cases in 2013 and 2014. Incidence is highest among infants, but is increasing in older children and adolescents, who now comprise the highest proportion of cases. Our current pertussis experience reflects a confluence of factors that illustrate the challenge of controlling a highly contagious disease that is often under-recognized with an imperfect vaccine in an era of increasing public distrust of vaccines.
Pertussis is a highly contagious, serious disease. Pertussis is caused by Bordetella pertussis, a fastidious gram-negative bacteria that infects the respiratory tract. Infection starts with a mild cough and runny nose that is difficult to distinguish from a common cold (the catarrhal stage). The bacteria produces a toxin, which paralyzes respiratory cilia, making it difficult to clear secretions. This leads to the severe cough that develops after 1 to 2 weeks. Classically, this is marked by episodes of rapid coughing that ends in the characteristic whoop as the lungs run out of air. This stage lasts for several weeks and can result in multiple complications, most commonly pneumonia. Pertussis is most severe in young infants; most (75%) will be hospitalized, and 1 in 100 will die. For older children and adults, the hospitalization rate is ~7%.
Pertussis is spread through respiratory droplets, and almost all (~90%) of susceptible people living with an infected person will acquire the disease when exposed. Because of how easily it spreads, almost everyone in a community needs to be immune to stop transmission. This requires high vaccination rates, especially in places where there are individuals (ie, young infants) who are not able to be fully vaccinated.
Pertussis can be difficult to recognize. Many infected people do not develop classic “whooping” symptoms, especially older children, adolescents, and adults who have partial immunity from previous vaccination or infection. They can present with a mild but persistent cough that may be diagnosed as other causes of chronic cough, such as asthma. Because infected people are contagious for up to 3 weeks after developing symptoms, those with unrecognized disease often unknowingly spread pertussis. A high index of suspicion is necessary when evaluating children with a prolonged cough without fever to promptly identify and treat suspected cases.
An imperfect vaccine. Preventing infection through vaccination is the best way to control pertussis within communities. Implementating current recommendations has significantly reduced disease incidence, but waning immunity after vaccination is challenging that success. A 2012 New England Journal of Medical (2012;367(11):1012-1019) article showed a significant reduction in vaccine efficacy within 5 years after completing the primary 5-dose series. This is likely related to the replacement of whole-cell pertussis vaccines with acellular pertussis vaccines in the 1990s. The whole cell elicits a strong and durable immune response. However, it also contains a large number of antigens leading to more side effects such as high fevers and prolonged crying.
In response to safety concerns, acellular vaccines targeting only 3 to 5 antigens related to our natural immune response were introduced. They are better tolerated and effectively prevent moderate to severe disease, but this may have come at the expense of more durable immunity.
Public distrust of vaccines. While waning immunity is a major driver of increased pertussis rates across the United States, vaccine hesitancy may also contribute to current trends. Indeed, in a recent California outbreak, cases were 8 times more likely to be unvaccinated. The 4-dose DTaP vaccination rate is 83% nationally, which remains below Healthy People 2020 targets, with significant variation across states. An estimated 1 in 8 undervaccinated children <2 years old have not been fully vaccinated due to parental choice. Evidence shows that clustering of nonmedical exemptions from school entry requirements is associated with incidence of pertussis. These trends are likely to continue as more parents delay or refuse some or all vaccines. In 11 states, the rate of nonmedical exemptions is >4%.
What can we do? While we do not have a perfect vaccine, implementing current vaccine recommendations is our best strategy to control transmission, reduce frequency of outbreaks and protect those most vulnerable to severe disease. The most severe cases of pertussis occur in young infants who have not yet received the full vaccine series; 75% of infants are infected by a household contact, most commonly the mother (33%) or father (16%) who may not know they are infected.
The Advisory Committee on Immunization Practices (ACIP) therefore recommends “cocooning:” immunizing everyone who has contact with young infants, including parents, grandparents, siblings, and out-of-home caregivers. ACIP also recommends that all pregnant women receive pertussis vaccine during each pregnancy, decreasing the chance of mother-to-baby exposure. More importantly, vaccinated mothers pass antibodies to the baby before birth, across the placenta, and after birth, through breast milk, providing protection before the infant can be immunized.
We have tools that are effective, but not perfect, to prevent a disease that can be a source of significant illness and death, especially for young infants. It is imperative we work to restore trust in these tools to protect our patients, families, and neighbors.
For information or to refer a patient to the Division of Infectious Diseases, call 215-590-2549. Information can be found at www.chop.edu/infectious-diseases.
The Vaccine Education Center at CHOP has a wealth of information for physicians and patient families. Please visit vaccine.chop.edu for information on all vaccines and for resources to use with families (print materials, videos, mobile app and more).