SAN DIEGO – The declining herd immunity to pertussis seen in the United States community may be related to withdrawal of the whole cell vaccine from the market about a decade ago, a study of more than 450,000 vaccinated patients from Kaiser Permanente Medical Center has shown.
Compared with their peers who had received at least one dose of whole cell vaccine, patients who had received only acellular vaccine had at least a tripling of the risk of acquiring the disease, lead author Maxwell Witt reported at IDWeek. The association was still present but weaker among patients who had received a total of six doses versus five.
©CDC
"Acellular pertussis vaccine offered significantly less protection when compared with the whole cell vaccine," said Mr. Maxwell Witt of Kaiser Permanente in San Rafael, Calif.
"Acellular pertussis vaccine offered significantly less protection when compared with the whole cell vaccine," commented Mr. Witt of Kaiser Permanente in San Rafael, Calif. "The risk of pertussis was mitigated, but not eliminated, by a sixth dose of pertussis vaccine, the Tdap [tetanus, diphtheria, and acellular pertussis] vaccine.
"The current generation of children is the first to have been vaccinated solely with the acellular pertussis vaccine," Mr. Witt said. "Our findings would predict a significant population of underprotected children in this group." Recent outbreaks of pertussis in the United States "had peak attack rates among those who are exactly in the same age group," he noted.
"The waning immunity associated with these outbreaks is clearly a call for development of more effective and durable pertussis vaccines," Mr. Witt said. "In the shorter term, strategies to prevent these outbreaks of pertussis could include either earlier or additional booster doses, and targeted vaccination programs to address insufficient immunity in outbreak situations."
The field of pertussis vaccination has undergone transition, with the introduction of the acellular vaccine in 1991 and retirement of the whole cell vaccine in 2001. "In 2010, we saw the largest epidemic of Bordatella pertussis in California in more than 50 years, and that epidemic subsequently spread across the United States," he said at IDWeek, the combined annual meetings of the Infectious Diseases Society of America, the Society for Healthcare Epidemiology of America, the HIV Medicine Association, and the Pediatric Infectious Diseases Society.
In a previous study, the investigators noted waning pertussis immunity among preadolescents (with a peak attack rate among 8- to 12-year-olds) but also robust immunity among adolescents (with a sharply lower attack rate among 13-year-olds) (Clin. Infect. Dis. 2010;54:1730-35), suggesting a possible role for the vaccine transition.
Mr. Maxwell Witt
In the new study, the investigators used electronic health records to identify vaccines administered and cases of laboratory-confirmed pertussis among Kaiser Permanente members aged 8-20 years in Northern California.
This age range was chosen to include patients who were old enough to have received whole cell vaccine and young enough to have received acellular vaccine, explained Mr. Witt. The search identified 465,059 patients, among whom there were 1,424 cases of pertussis.
A total of 253,005 patients had received all of their vaccine doses at Kaiser Permanente. Analyses restricted to this group showed that among patients who had received five total doses of vaccine, the pertussis attack rate was 786/100,000 for those who had received only acellular vaccine, compared with 92/100,000 for those who had received at least one dose of whole cell vaccine. The difference corresponded to an 8.57-fold higher risk for the former group (P less than .0001).
Similarly, among patients who had received six total doses of vaccine, the attack rate was 378 vs. 106/100,000 for those who received only acellular vaccine compared with those who had received at least one dose of whole cell vaccine. Here, the difference amounted to a smaller but still significant 3.55-fold higher risk.
The study findings were essentially the same when analyses were based on all patients, including those who had received at least some doses of vaccine outside of Kaiser Permanente, with respective 6.76- and 2.46-fold higher risks for patients given only acellular vaccine, depending on the total number of doses.
Mr. Witt disclosed no relevant financial conflicts.