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ABSTRACT. Objectives. To determine the reactogenicity and immunogenicity of a fourth dose of 2 threecomponent acellular pertussis vaccines combined with diphtheria-tetanus-acellular pertussis (DTaP) when administered at preschool age to children primed in infancy with 3 doses of the same DTaP and who had received a diphtheria-tetanus (DT) dose at the age of 12 months.Setting. Local health units of 4 Italian regions. Study Design. Three thousand five hundred twentytwo children, who had been randomized in the first year of life to be immunized with a DTaP vaccine by either SmithKline Beecham or Chiron Biocine, were offered a booster of the same vaccine or, if refusing, a DT vaccine at the age of 5 to 6 years. Families of children were aware of the vaccine administered. The occurrence of adverse events was compared between the children who received a DTaP booster and those boosted with a DT only. Antibody titers to pertussis vaccine components (pertussis toxin, filamentous hemoagglutinin, and pertactin) were determined on 558 paired sera taken before and 30 days after the DTaP booster administration.Results. Four episodes of temperature >39.5°C, 2 in each DTaP group, were recorded. Fever >38°C occurred infrequently in both DTaP and DT recipients (DTaP range: 2.5%-2.8%; DT range: 0%-4.8%), as did irritability (DTaP range: 10.1%-11.7%; DT range: 7.4%-12.6%). The frequency of local reactions was significantly higher for DTaP recipients (range: 44.0%-52.8%), with respect to DT recipients (range: 29.5%-44.4%). Extensive local reactions were observed in 1.2% of DTaP recipients and in .5% of DT recipients. Both DTaP vaccines induced high antibody titers against pertussis toxin, filamentous hemoagglutinin, and pertactin, with an increase of >10 times the prebooster geometric mean titers.Conclusions. A booster dose of DTaP at preschool age in children primed with the same acellular pertussis vaccine is safe and immunogenic. However, the frequency of local reactions is higher compared with that following primary immunization and with that following booster with DT only, and parents should be informed of the potential for these reactions to occur. Pediatrics 2001; 107(2). URL: http://www.pediatrics.org/cgi/content/full/ 107/2/e25; pertussis vaccine, adverse effects, immunoresponse, booster dose, children.ABBREVIATIONS. DTaP, diphtheria-tetanus-acellular pertussis; DT, diphtheria-tetanus; SB, SmithKline Beecham; CB, Chiron Biocine; PT, pertussis toxin; FHA, filamentous hemoagglutinin; PRN, pertactin; IgG, immunoglobulin G; ELISA, enzyme-linked immunosorbent assay; EU, ELISA units; MLD, minimal level of detection; PT CHO, antibodies neutralizing pertussis toxin on Chinese hamster ovary cells; GMT, geometric mean titer; RR, relative risk; CI, confidence interval. R ecent studies, including clinical trials, have demonstrated that acellular pertussis vaccines are efficacious and have a lower reactogenicity than whole-cell vaccines. [1][2][3][4][5] These findings have led various countries to use acellular vaccines for primary immu...

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