Pertussis Vaccination and Increase in Cases

Despite an extensive and widely available childhood vaccination program, pertussis remains a poorly controlled vaccine-preventable disease in the United States. Infants have higher rates of pertussis compared with children in other age groups, however, recent data from the National Notifiable Diseases Surveillance System show an increase in the number of pertussis cases among children 7 to 10 years of age. In 2010, US children 7 to 10 years of age had the second highest incidence of pertussis.

Also in 2010, California reported >9000 pertussis cases, the most in 60 years. Concern about the number of cases in California, and the increased number of cases among children 7 to 10 years of age, has generated a large-scale assessment of the current pertussis childhood vaccination program.

The investigation aimed to evaluate the association between pertussis and receipt of 5 diphtheria, tetanus, and 5 acellular pertussis (DTaP) doses by time since the fifth DTaP dose. Results were reported in the Journal of the American Medical Association [2012;308(20):2126-2132].

The evaluation was conducted in 15 counties in California. Pertussis cases (n=682) among children 4 to 10 years of age reported from January through December 14, 2010 were classified as suspected, probable, and confirmed. Controls were 2016 children in the same age group who received care from clinicians reporting the cases. Vaccination histories were obtained from medical records and immunization registries.

The primary outcomes were (1) odds ratios (ORs) for the association between pertussis and receipt of the 5-dose DTaP series and (2) ORs for the association between pertussis and time since completion of the 5-dose DTaP series (<12 months, 12 to 23 months, 24 to 35 months, 36 to 47 months, 48 to 59 months, and ≥60 months).

Of the 682 cases included in the evaluation, 61.3% (n=418) were confirmed, 9.4% (n=64) were probable, and 25.5% (n=174) were suspected cases. Cases were more likely to be unvaccinated compared with the controls (7.8% [n=53] vs 0.9% [n=19]; P<.001) and female (55.0% [n=375] vs 47.5% [n=958]; P=.001). Cases were older than controls (9 vs 7 years of age, respectively; P<.001). The majority of both cases and controls received their fifth DTaP dose at 4 years of age.

Among the cases, 7.8% (n=53) had not received any pertussis-containing vaccines compared with 0.9% (n=19) of controls. Compared with controls, children with pertussis had lower odds of having received all 5 doses of DTaP (OR, 0.11; 95% confidence interval [CI], 0.06-0.21 [estimated vaccine effectiveness (VE), 88.7%; 95% CI, 79.4%-93.8%]).

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 (2.8% [n=19] vs 16.5% [n=354], respectively; OR, 0.02; 95% CI, 0.01-0.04 [estimated VE, 98.1%, 95% CI, 96.1%-99.1%]).

This association was evident with longer time since vaccination, with ORs increasing with time since the fifth dose. At ≥60 months (33.9% [n=231] cases and 14.3% [n=288 controls]), the OR was 0.29 (95% CI, 0.15-0.54 [estimated VE, 71.2%; 95% CI, 45.8%-84.8%]). Accordingly, the estimated VE declined each year following receipt of the fifth dose of DTaP.

In conclusion, the researchers stated, “Among children in 15 California counties, children with pertussis, compared with controls, had lower odds of having received the 5-dose DTaP series; as time since last DTaP dose increased, the odds increased, which is consistent with a progressive decrease in estimated vaccine effectiveness each year after the final dose of pertussis vaccine.”