Epidemiological Trends in Pertussis Reveal Limitations of Acellular Vaccines

An examination of 17 years of data reveals that children who received the combination acellular vaccine Tdap were vulnerable to pertussis much earlier than expected. Researchers are hopeful that new vaccines will extend protection against the disease.

Pertussis, also known as whooping cough, is a bacterial infection that continues to be diagnosed in individuals nationwide even though vaccination has been offered for decades. In an effort to discover patterns of the disease in recent years, as well as delve into the characteristics of those most vulnerable to it, a team of scientists from the Centers for Disease Control and Prevention (CDC) examined case reports from January 1, 2000, through December 31, 2016.

During the 17-year period studied, 339,420 cases of pertussis were reported through the National Notifiable Diseases Surveillance System; nearly 10% of them required hospitalization, and 294 patients, or slightly more than .1%, died. The vast majority—nearly 89%—of fatal cases occurred in infants less than 1 year old, even though infants made up only 15.1% of total reported cases. Children aged 1 to 6 comprised 16.8% of cases, children aged 7 to 10 accounted for 15%, and adolescents aged 11 to 18 made up 27.7%. Nineteen- to 29-year-old patients were responsible for only 3.3% of the total cases. Subjects aged 30 to 64 comprised 19.7% of the total, and those 65 or older accounted for just 2.4%.

Whooping cough, which can last up to 2 weeks, often starts out mildly and can mimic a cold. Symptoms include an occasional cough, runny, nose, and fever, although in babies symptoms may also include short periods of not breathing. Untreated, the condition can progress to aggressive coughing fits that lead to vomiting and exhaustion. Sufferers can cough so violently that they literally can’t catch their breath; when they finally do, they make a high-pitched sound that gives the disease its name.

The highest average yearly incidence of pertussis in this study, by far, occurred in infants, who are most vulnerable to serious complications. Roughly half of all infants with pertussis need hospitalization; the disease can deprive babies of oxygen to such a degree that they suffer seizures, brain damage, and even death. The scientists also noticed some intriguing patterns when it came to the numbers of older children diagnosed with pertussis. Excluding infants, in the early years of the study--from 2000 to 2006—children between the ages of 11 and 18 were the most likely group to have pertussis. But from 2007 to 2014, children from ages 7 to 10 were the group most likely to have it. In the final 2 years of the study, 2015 and 2016, rates of the disease in kids aged 11 to 18 slightly surpassed those in kids aged 7 to 10. What was going on?

According to the study team, one of the main culprits behind the differences in the most vulnerable age groups over the years was a change to the recommended pertussis vaccines. Because of safety concerns about whole-cell vaccines, acellular vaccines were developed and first put into use in the 1990s. The combination vaccine known as Tdap (tetanus, diphtheria, and pertussis) was recommended for adolescent use beginning in 2005. But the investigators noticed something interesting: as the children who received the acellular vaccine got older, their protection against pertussis began to wane much earlier than expected. For example, around 2008 there was a noticeable and unexpected rise in pertussis cases among children between 7 and 10 years old, “a group that should have been well-protected by increasing Tdap coverage,” the authors wrote in their report.

“This was the first cohort of kids that were fully vaccinated with acellular vaccines,” Tami Skoff, MS, an epidemiologist with the CDC and an author of the study, told Contagion®. According to Ms. Skoff, there’s a lot of work being done right now to develop new vaccines that are more effective. “I think the next generation of vaccines--what developers will really be targeting is a longer duration of protection,” she said. She added that although waning protection against pertussis means vaccinated people can still get the disease, it’s important to note that vaccination may result in milder cases. Other factors possibly contributing to the rise in diagnosed cases include molecular changes to the pertussis organism and the use of more sensitive tests.

Overall, the study team found that the number of cases of pertussis has risen slowly and steadily in the past 17 years, with peaks every 2 to 5 years; however, the disease peaks in different places at different times. Additionally, rates of the disease are not uniform nationwide. New England, for example, has had lower rates of the disease than the Pacific states. Ms. Skoff said some of these variations could be caused by differences in surveillance techniques or testing practices.

For clinicians in all parts of the United States, Ms. Skoff urges vigilance when it comes to patients complaining of a cough. “Include pertussis in the diagnostic differential,” she said. “An early diagnosis and treatment of pertussis is really key.” And don’t ignore pregnant women. As babies younger than 2 months can’t be given the pertussis vaccine, mothers may be a bulwark against the disease. Current guidelines recommend vaccination during every pregnancy. “Maternal immunization during pregnancy is really key,” said Dr. Skoff. “This is the group that we really want to focus on.” Currently, only about half of all pregnant women get the Tdap vaccine.

Ms. Saloman is a health writer with more than 20 years of experience working for both consumer-and physician-focused publications. She is a graduate of Brandeis University and the Medill School of Journalism at Northwestern University. She lives in New Jersey with her family.