P<0.05 was considered significant. Results: 725 children were enrolled in the study. Geometric mean titers for IgG that showed a slight decease after 2 months of age and increased distinctly in children aged 72 months. The frequency of the individuals whose IgG was above the determined cut-off (derived

from mean+2SD) was observed in 1% of the 2, 4, and 6-month-old infants, 6% of the 12 and 18-month-olds Inhibitors,research,lifescience,medical and 12% of the 6-year -old children. Positive IgA titers were detected in 5, 9, 6, 23, 11, and 8% of children aged 2, 4, 6, 12, 18, and 72 months, respectively. Conclusion: Since a considerable percentage of children had high levels of anti-pertussis IgG antibodies (≥2 SD), positive anti-pertussis IgA, and most importantly an increased level of anti-pertussis IgG geometric mean titer at 6 years of age, further Inhibitors,research,lifescience,medical investigations regarding the protection provided by the presently used pertussis vaccine seems necessary. Key Words: Pertussis, Whooping cough, Pertussis vaccine

Introduction Reports from the World Health Organization (WHO) reveal that 17.6 million Inhibitors,research,lifescience,medical persons contracted whooping cough in 2003 resulting in 279000 deaths; however, some authorities estimate the real figures of cases with pertussis to be around 50 million with 90% living in developing countries.1 According to the report of WHO Regional Office for the Eastern Mediterranean, 19000 deaths have occurred in under 5-year-old children because of pertussis. 464 patients were reported to have pertussis in Iran, in 2010.2 Immunization against pertussis is included in the expanded program of immunization developed by the WHO and Inhibitors,research,lifescience,medical has been implemented in Iran since 1950. Whole cell pertussis vaccine is used in meanwhile combination with diphtheria and tetanus toxoids (as DwPT) to vaccinate infants and children against these three deadly diseases.1 In spite of a coverage of >95% for DwPT, the incidence Inhibitors,research,lifescience,medical of pertussis seems to be increasing during recent years. Young children continue to contract whooping cough and are placed at risk for complications and sometimes sellckchem mortality associated with this vaccine preventable disease.1,3,4 Occurrence of pertussis in vaccinated

individuals has raised questions about the protection afforded by the whole cell vaccine; numerous researchers have suggested different approaches for assessment of vaccine efficacy including estimating Drug_discovery the prevalence of pertussis in vaccinated populations.1,5-8 Since pertussis may mimic other respiratory diseases such as adenovirus, influenza and mycoplasma infections, resulting in the so-called “pertussis-like” syndrome, relying solely on the clinical presentation would not be a true measure for documenting the disease, as many cases would be clinically mislabeled as pertussis. Current diagnostic procedures include culture, polymerase chain reaction (PCR), and a rise in antibodies through enzyme-linked immuno-assay (ELISA).