Outbreak of suspected pertussis in Kaltungo, Gombe state, Northern Nigeria, 2015: the role of sub-optimum routine immunization coverage

Introduction : pertussis is a vaccine preventable disease. However, outbreaks have been documented among vaccinated populations. In December 2015, we investigated a pertussis outbreak in Kaltungo, Nigeria to identify determinants of infection and institute control measures. Methods : we enrolled 155 cases and 310 controls in an unmatched case-control study. We defined cases as residents of Kaltungo with paroxysmal or whooping cough lasting 2 weeks with or without vomiting and randomly selected neighbourhood controls. Using structured questionnaire, we collected data on socio-demographics, clinical and risk factors and analyzed using Epi-info7. We collected twelve nasopharyngeal swabs for laboratory analysis using Polymerase Chain Reaction. Results : median age was 24 months (range 1 - 132 months) for cases and 27 months (range 1 – 189 months) for controls; female cases and controls were 86 (55.5%) and 150 (48.4%) respectively. A total of 83 (56.6%) cases were within age group 12 - 59 months. Age-specific-attack-rate was 83/1,786 (4.7%); Age-specific-case-fatality-rate was 21/83 (25.3%); Age-specific-proportional-mortality-ratio was 21/24 (87.5%). Vaccination records revealed 61 (39.4%) zero doses and 30.1% Pentavalent dropout. With bivariate analysis, contact with a case (OR = 8.8; 95% CI = 5.4 - 14.3), parental refusal of vaccination (OR = 20.2; CI- 10.2 - 29.8), mother having informal education (OR = 5.4; CI-3.4-8.6) and being a Muslim (OR=3.9, CI = 2.5 - 6.0) were significantly associated with having pertusis, while receiving Pentavalent 3 (OR = 0.3, CI - 0.2 - 0.4,) was protective. Multivariate analysis revealed parental refusal (adjusted OR = 27.8; CI = 8.8 - 87.7), contact with a case (AOR = 7.9, CI = 4.3 - 14.7, P = 0.000), belonging to the Muslim faith (AOR = 2.0; CI = 1.1 - 3.5) and having mothers with informal education only (AOR = 4.7, CI - 2.6 - 8.4) as independent predictors of pertussis infection. Conclusion : sub-optimal vaccination due to parental refusal and informal education of mothers were major determinants of pertussis infection. We conducted awareness campaigns using key immunization messages targeted at the informally educated persons. We mobilized resources for case management, contact vaccination and health education in public gatherings, worship places and schools.