It’s news that’s certain to ignite debates over vaccines again: researchers say that the childhood immunization against whooping cough fades substantially over time, leaving even fully vaccinated children vulnerable to infection. Weakened protection may be a major contributor to the current outbreak of whooping cough, or pertussis, in the U.S. — the largest in decades — the new study suggests.
In the report, published in the New England Journal of Medicine, Dr. Nicola Klein and her colleagues at the Kaiser Permanente Vaccine Study Center found that the efficacy of the pertussis vaccine declined sharply after children received the last of the five recommended childhood shots, increasing their risk of acquiring the disease by 42% each year. In other words, the study showed, a vaccine that was 95% effective to start would be only 71% effective five years later.
The findings help explain the recent resurgence of pertussis, which has already affected more than 26,000 people this year. Concerns over the vaccine’s effectiveness arose in 2010, when a large outbreak of the disease hit California puzzling scientists, since most of the affected school-aged children had been properly vaccinated. That year, the nation saw 27,550 cases of pertussis overall, the highest since 1959. Another outbreak in Washington state earlier this year only heightened the concern. Although 2012 should have seen a natural downswing in the cyclical pattern of the disease, it’s on track to be the worst year yet.
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To better understand the increasing rates of disease, Klein and her group turned to Kaiser’s population of managed care members, focusing on children ages 4 to 12, who were enrolled in Kaiser’s health plan in northern California in 2006 to 2011. The researchers compared 277 fully vaccinated children who tested positive for the pertussis, which is caused by the Bordetella pertussis bacterium, witht two control groups: one that included 3,318 children who tested negative for the microbe and another group of 6,086 children who were not tested for the disease. The second control group allowed the scientists to adjust for any lifestyle or behavioral factors that might account for some families’ better access to testing or health care services.
All children received all five doses of the pertussis vaccine, known as DTaP (for diphtheria, tetanus and acellular pertussis). The researchers looked specifically at the time between the last dose — given between ages 4 and 6 — and the onset of disease in children who acquired pertussis, and found that indeed, they were more likely to have experienced more lag time. Children who tested positive for pertussis had received their last DTaP dose 1,699 days earlier, compared with 1,028 days for those testing negative.
“We found a substantial waning in protection in the five years after the fifth dose,” says Klein, adding that the findings do not suggest the current vaccine is useless. “We also found was that some protection is better than no protection. The existing DTaP is safe and effective, but protection just doesn’t last as long as we’d like.”
Although the vaccine’s effectiveness weakens over time, the protection it offers is still critical for maintaining enough population-based immunity to safeguard infants, who are too young to be vaccinated against infection. Health experts stress that parents should not to stop vaccinating their kids just because the immunizations appear to lose their effectiveness.
The DTaP vaccine, which uses purified snippets of the pertussis microbe to trigger immunity, was first introduced in the U.S. in 1991. It replaced the older immunization developed in the 1940s, which used whole, killed B. pertussis cells. Adverse reactions to that vaccine, including high fever and some neurological symptoms, led to the switch.
The problem was that health officials began to see an increase in cyclical outbreaks of pertussis, occurring every three to five years, among children who were properly vaccinated using DTaP. As these reports, suggesting a weakening of DTaP’s effectiveness, began to build, the Centers for Disease Control and Prevention (CDC) recommended in 2005 that people 11 years old or older receive a sixth dose, or booster, with the Tdap vaccine, which is designed for teens and adults, to bolster their defenses against the bacterium.
Was this waning efficacy known at the time that DTaP was licensed? “No,” says Dr. Tom Clark, medical epidemiologist at the CDC. “Vaccines are studied extensively in thousands of kids before they are licensed. But we never really know what is going to happen until we implement immunizations on a public health scale. We never expected we would be doing these kinds of vaccine effectiveness studies on vaccines that we’ve been using for 20 years.”
Vaccine studies evaluated by health experts before licensing often don’t follow participants for very long, however, says, which means that the true effectiveness of immunizations over time is often not learned until they become part of the vaccine schedule.
There were hints that the newer, acellular version of the vaccine was slightly less effective than the older shot, says Dr. Kathryn Edwards, chair of pediatrics at Vanderbilt University and a member of the American Academy of Pediatrics’ Committee on Infectious Diseases, who was part of the initial studies on the DTaP vaccine, but the tradeoff in fewer adverse reactions was worth the switch. “It was clear in those studies conducted in the 1990s that the whole cell vaccine was still better than the acellular vaccine,” she says. “But by and large the acellular was 85% effective, and was much safer.”
Health experts point out that the current study did not look at DTaP’s effectiveness per se; rather, it assessed the risk of pertussis among a vaccinated group of children. The burden of disease in the Kaiser population could have been higher than average, notes Edwards. In that area of northern California, about 11% of children either aren’t vaccinated at all or only partially immunized against pertussis, and that may increase children’s exposure to the bacterium. Couple that with the fact that the DTaP vaccine isn’t 100% effective — no vaccine is — conferring only about 85% protection, and it’s not that surprising that immunity starts to falter.
Nevertheless, the findings reveal a weakness in the pertussis vaccine that public health experts will have to address. It’s not clear how much added protection the Tdap booster provides, but for now, it’s a stopgap measure to protect against infection and spread of the disease. “We do think that the long-term answer is to find a new vaccine that provides long-lasting immunity,” says Klein.