Investigators set out to find out why pertussis rates in the United States have been rising steadily despite the availability of a vaccine.
New research published in Science Translational Medicine pinpoints gaps in the effectiveness of the pertussis vaccine and suggests that schoolchildren need to be the main focus if the United States is to reverse the upward trend of pertussis cases.
Also known as whooping cough, pertussis has been vaccine-preventable since the 1940s, and according to the US Centers for Disease Control and Prevention (CDC), about 85% of children ages 19 to 35 months receive a vaccination against the disease each year. Yet, rates of pertussis have been rising steadily since the 1970s. In fact, according to the CDC, 20,762 cases of pertussis were reported in the United States in 2015, and 3 infants died of the disease that same year.
To this end, investigators from the University of Michigan, University of Georgia, Queen’s University in Canada, and the University of Versailles St-Quentin-en-Yvelines, in France, sought to figure out exactly what is going wrong with regard to vaccine protection against pertussis.
“Our main motivation when we started this study was to better understand the nature of immunity conferred by pertussis vaccines—a hotly debated topic in pertussis epidemiology,” Matthieu Domenech de Celles, PhD, of the University of Versailles St-Quentin-en-Yvelines told Contagion®. “In this context, we sought to test a number of hypotheses about pertussis vaccines in a systematic and rigorous way, leaving our prior expectations aside.”
To study the problem, the researchers pulled 16 years of data from Massachusetts, stretching from 1990 to 2005. Massachusetts was chosen because it has a robust disease surveillance program and sufficient data to track vaccinations and incidences of the disease.
Those data, along with age-specific daily contact rates from a 2008 study in Great Britain, were used to develop a mathematical model of disease transmission.
The team probed 3 potential causes: 1) that the vaccine simply didn’t work in some patients; 2) that vaccine-induced protection waned over time; and, 3) that the protection provided by the vaccine was “leaky” or imperfect.
After crunching the numbers, the model suggested that it was a combination of the latter 2: that the vaccine works imperfectly, and in some patients, its effectiveness wanes over time.
Specifically, the model suggested that 10% of patients see their protection disappear completely within a decade of taking the recommended course of vaccinations. However, in 55% of patients, the protection remains for the rest of their lives.
When a patient is infected post-vaccination, the researchers found no difference in the rates of transmission—meaning the vaccine doesn’t make the disease less transmittable if a person becomes infected. However, the team suggested that such cases are more likely to go undetected.
Domenech de Celles said he believes the study has implications for physicians and patients nationwide, not just in Massachusetts.
“Although our study was conducted in Massachusetts, we believe our results also apply in other parts of the United States,” he said. In fact, most US states have experienced a resurgence of pertussis comparable to that seen in Massachusetts.
Notably, the researchers also found that children are the most likely transmitters of the disease. “More generally, our results indicate a key role for children and adolescents and at most a minor role for adults in pertussis transmission, perhaps due to the differences in the frequency of contacts at different ages and the assortative structure of the contact network,” the authors wrote.
The authors also stated that public health officials should keep this in mind when designing vaccine schedules.
Given that the vaccine can have different levels of effectiveness and longevity in different people, the investigators suggested that a targeted campaign of booster shots would be the only way to ensure eradication of the disease.