The impact of additional pertussis vaccine doses on disease incidence in children and infants

Background: Pertussis remains a cause of considerable morbidity in children worldwide. Due to the resurgence of the disease, two vaccine doses for schoolchildren were added to the routine Israeli schedule. In 2005 a 5th dose was introduced for second-graders (aged 7-8), and in 2008 an additional catch-up dose in the eighth grade (13-14 year-olds). Methods: Population-based epidemiologic study of pertussis in the Jerusalem district. Results: 1736 pertussis cases were reported from 1990 to 2009. The pertussis incidence rates increased sharply from 2.6/100,000 in 1990, to 10/100,000 in 2000, peaking at 28.8/100,000 in 2006, then declining to 22/100,000 in 2008 and to 15.7 in 2009 (2006 vs. 2009, p= 0.0001). Most cases (74.4%, 1134/1524 during 1998-2009) were under 20 years. Infants under one year had the highest average incidence rate (72.3/100,000; 12.5% of cases); specifically those under 6 months (84.3% of cases under one year). The case distribution among 1-4, 5-9, 10-14, and 15-19 year-olds was: 11%, 18%, 24.1%, and 8.9%. The vaccination status (age-appropriate) was: unvaccinated - 19.2%, partially vaccinated - 7.6%, and fully vaccinated - 73.2%. The overall hospitalization rate was 5.4%; infants - 33.5%. Household transmission occurred in 16.1% of cases.The two age groups showing significant decline were children aged 5-9 (61.5% reduction) and 10-14 years (73.9% reduction); there is as yet no significant decline in other age groups. Conclusions: The recent marked decline in pertussis incidence among the 5-14 year-olds is encouraging. Young infants still constitute a significant disease burden, and the incidence in this age group should be followed closely.