Effectiveness of Haemophilus influenzae type b Conjugate vaccine introduction into routine childhood immunization in Kenya

Abstract

Context: Haemophilus influenzae type b (Hib) conjugate vaccine is not perceived as a public health priority in Africa because data on Hib disease burden and vaccine effectiveness are scarce. Hib immunization was introduced in Kenyan infants in 2001.

Objective: To define invasive Hib disease incidence and Hib vaccine program effectiveness in Kenya.

Design, setting, and patients: Culture-based surveillance for invasive Hib disease at Kilifi District Hospital from 2000 through 2005 was linked to demographic surveillance of 38,000 children younger than 5 years in Kilifi District, Kenya. Human immunodeficiency virus (HIV) infection and Hib vaccination status were determined for children with Hib disease admitted 2002-2005.

Interventions: Introduction of conjugate Hib vaccine within the routine childhood immunization program at ages 6, 10, and 14 weeks beginning November 2001.

Main outcome measures: Incidence of culture-proven Hib invasive disease before and after vaccine introduction and vaccine program effectiveness.

Results: Prior to vaccine introduction, the median age of children with Hib was 8 months; case fatality was 23%. Among children younger than 5 years, the annual incidence of invasive Hib disease 1 year before and 1 and 3 years after vaccine introduction was 66, 47, and 7.6 per 100,000, respectively. For children younger than 2 years, incidence was 119, 82, and 16 per 100,000, respectively. In 2004-2005, vaccine effectiveness was 88% (95% confidence interval, 73%-96%) among children younger than 5 years and 87% (95% confidence interval, 66%-96%) among children younger than 2 years. Of 53 children with Hib admitted during 2002-2005, 29 (55%) were age-ineligible to have received vaccine, 12 (23%) had not been vaccinated despite being eligible, and 12 (23%) had received 2 or more doses of vaccine (2 were HIV positive).

Conclusions: In Kenya, introduction of Hib vaccine into the routine childhood immunization program reduced Hib disease incidence among children younger than 5 years to 12% of its baseline level. This impact was not observed until the third year after vaccine introduction.

Figure 1

Age-frequency of 190 episodes of invasive H. influenzae type b disease admitted to KDH before Hib vaccine introduction (1994–2001), and cumulative percentage of cases with increasing age. Vertical bars show the frequency of Hib cases in each 2 month stratum of age. The solid line shows the cumulative percentage of cases by age. Hib was diagnosed by CSF, pleural fluid or blood culture, or by CSF Hib antigen. Twenty-one (11%) of 190 cases were ≥ 36 months in age (not illustrated). The eldest child was 8 years old

Figure 2

Incidence of invasive disease caused by H. influenzae type b and S. pneumoniae among children aged <5 years in Kilifi DSS area in each year of the study Grey and black lines indicate incidence of invasive pneumococcal and Hib disease respectively. Error bars on the incidence points are 95% confidence limits.