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Children in Calif. pertussis outbreak less likely to receive five-dose DTaP series
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Among children in 15 California counties, children with pertussis exhibited lower odds of having received all five doses of the diphtheria and tetanus toxoids and acellular pertussis vaccine series, according to study results.
Nevertheless, their odds increased as the time since last DTaP dose lengthened — consistent with a progressive decrease in estimated vaccine effectiveness each year after the final dose of DTaP vaccine.
“In 2010, California experienced its largest pertussis epidemic in more than 60 years; more than 9,000 pertussis cases were reported and 10 infants died,” Lara K. Misegades, PhD, MS, of the CDC, and colleagues wrote. “Concordant with national trends, a substantial burden of disease (67.9 cases per 100,000) occurred in 7- to 10-year-olds despite high DTaP coverage. Concern about the number of cases in California and the increasing burden of pertussis among 7-to 10-year-olds prompted a large-scale assessment of the long-standing pertussis childhood vaccination program.”
To evaluate the association between pertussis and receipt of five DTaP doses by time since the fifth DTaP dose, Misegades and colleagues conducted case-control evaluations of 15 California counties.
Six hundred eighty-two suspected, probable and confirmed pertussis cases among children aged 4 to 10 years reported from January to Dec. 14, 2010, were compared against 2,016 control children in the same age group who received care from the clinicians reporting the cases. In addition, vaccination histories were obtained from medical records and immunization registries.
According to study results, 53 (7.8%) of pertussis cases had not received any pertussis-containing vaccine vs. 19 (0.9%) of the control group. Compared with controls, children with pertussis also exhibited a lower odds of having received all five doses of DTaP (OR=0.11; 95% CI, 0.06-0.21 [estimated vaccine effectiveness, 88.7%; 95% CI, 79.4-93.8]).
“Although a small proportion of children in California were susceptible to pertussis due to their unvaccinated status, our findings suggest that waning of immunity following DTaP vaccination may have resulted in a much larger pool of susceptible individuals,” Misegades and colleagues wrote. “In periods of increased pertussis transmission, the burden of disease attributable to the vaccinated but susceptible population is high.”
When children were categorized by time since completion of the DTaP series, using an unvaccinated reference group, children with pertussis compared with controls were less likely to have received their fifth dose within the prior 12 months — 19 (2.8%) in pertussis cases vs. 354 (17.6%) in the control group (OR=0.02; 95% CI, 0.01-0.04 [estimated vaccine effectiveness, 98.1%; 95% CI, 96.1- 99.1]).
According to Misegades and colleagues, “The increasing incidence of pertussis, changing epidemiology, and demonstrated decline in the estimated DTaP [vaccine effectiveness] over time have raised concerns about the current US pertussis vaccine program and may prompt consideration of alternative schedules.”
Disclosure: The researchers report no relevant financial disclosures.
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