A large Kaiser Permanente study paints a nuanced picture of the acellular pertussis vaccine, with more cases occurring in fully vaccinated children, but the highest risk of disease occurring among the under- and unvaccinated.
Among nearly half a million children, the unvaccinated were 13 times more likely to develop pertussis than fully vaccinated children,of Kaiser Permanente Northern California in Oakland and colleagues wrote in . But 82% of cases occurred in fully vaccinated children and just 5% in undervaccinated children – and rates increased in both groups the farther they were in time from the last vaccination.
“Within our study population, greater than 80% of pertussis cases occurred among age-appropriately vaccinated children,” the team wrote. “Children who were further away from their last DTaP dose were at increased risk of pertussis, even after controlling for undervaccination. Our results suggest that, in this population, possibly in conjunction with other factors not addressed in this study, suboptimal vaccine efficacy and waning [immunity] played a major role in recent pertussis epidemics.”
The results are consistent with several prior studies, including one finding that the odds of the disease increased by 33% for every additional year after the third or fifth DTaP dose ().
The current study comprised 469,982 children aged between 3 months and 11 years, who were followed for a mean of 4.6 years. Over the entire study period, there were 738 lab-confirmed pertussis cases. Most of these (515; 70%) occurred in fully vaccinated children. Another 99 (13%) occurred in unvaccinated children, 36 (5%) in undervaccinated children, and 88 (12%) in fully vaccinated plus one dose.
In a multivariate analysis, the risk of pertussis was 13 times higher among the unvaccinated (adjusted hazard ratio, 13) and almost 2 times higher among the undervaccinated (aHR, 1.9), compared with fully vaccinated children. Those who had been fully vaccinated and received a booster had the lowest risk, about half that of fully vaccinated children (aHR, 0.48).
Risk varied according to age, but also was significantly higher among unvaccinated children at each time point. Risk ranged from 4 times higher among those aged 3-5 months to 23 times higher among those aged 19-84 months. Undervaccinated children aged 5-7 months and 19-84 months also were at significantly increased risk for pertussis, compared with fully vaccinated children. Children who were fully vaccinated plus one dose had a significantly reduced risk at 7-19 months and at 19-84 months, compared with the fully vaccinated reference group.
“Across all follow-up and all age groups, VE [vaccine effectiveness] was 86% ... for undervaccinated children, compared with unvaccinated children,” Dr. Zerbo and associates wrote. “VE was even higher for fully vaccinated children [93%] and for those who were fully vaccinated plus one dose [96%].”
But VE waned as time progressed farther from the last DTaP dose. The multivariate model found more than a 100% increased risk for those whose last DTaP was at least 3 years past, compared with less than 1 year past (aHR, 2.58).
The model also found time-bound risk increases among fully vaccinated children, with a more than 300% increased risk among those at least 6 years out from the last DTaP dose, compared with 3 years out (aHR, 4.66).
The results indicate that other factors besides adherence to the recommended vaccine schedule may be at work in recent pertussis outbreaks.
“Although waning immunity is clearly an important factor driving pertussis epidemics in recent years, other factors that we did not evaluate in this study might also contribute to pertussis epidemics individually or in synergy,” Dr. Zerbo and associates wrote. “Results from studies in baboons suggest that the acellular pertussis vaccines are unable to prevent colonization, carriage, and transmission. If this is also true for humans, this could contribute to pertussis epidemics. The causes of recent pertussis epidemics are complex, and we were only able to address some aspects in our study.”
The study was funded by Kaiser Permanente Northern California, the National Institutes of Health, and in part by a National Institute of Allergy and Infectious Diseases grant. One coauthor reported receiving research grant support from Sanofi Pasteur, Novartis, GlaxoSmithKline, Merck, MedImmune, Pfizer, and Dynavax for unrelated studies; the other authors reported no relevant financial disclosures.
SOURCE:Zerbo O et al.