Bordetella pertussis: Vaccine Coverage and Immune Status among Children Under Five Years Old in Sana’a City, Yemen

Aims: This study was made to evaluate the immune response to pertussis among children under five years old by measuring the level of circulating Immunoglobulin G (IgG) antibodies against pertussis toxin (IgG-PT) after immunization with the primary series of DPT (DPT1-3) and then determining the coverage rates of universal childhood vaccines.

Study Design: Cross-sectional laboratory study.

Place and Duration of Study: Department of Medical Microbiology and Clinical Immunology, Faculty of Medicine and Health Sciences, Sana’a University, between June and October 2010.

Methodology: A total of 345 children were randomly selected and investigated for universal childhood vaccination coverage rates; of these, 273 children who had received 3 full doses of DPT were studied for their pertussis immunological status. Blood samples were collected from the latter group and then tested for levels of IgG-PT by ELISA method. For assessment IgG-PT levels more than 24 units/ml were considered protected against pertussis. Data were analyzed according to gender and age groups.

Results: The coverage rate of pertussis vaccine was 79.1%; only 71.4% of vaccinated children responded to the vaccine with mean level of 33 U/ml. A statistically significant difference was observed with respect to sero-protective IgG-PT between males and females (63.8% and 84.8% respectively, with χ2 =15, p=0.0001). Also, a statistically significant difference was observed with respect to sero-protective IgG-PT in different age groups, with older children (>48 months) having a higher rate of antibodies than younger (13-24 months) (90% and 59.1% respectively, with χ2 =3.87, p=.04).

Conclusion: We conclude that a considerable proportion of vaccinated children with a normal immune status were not serologically immune to pertussis. They remain to be reconsidered for either revaccination or booster doses due to lack of or inadequate response. Also, the rates of vaccine coverage for the main universal childhood vaccines are low.