New Vaccine Introduction and Childhood Vaccination Timeliness in Two Urban, Informal Settlements in Nairobi, Kenya.

New vaccine introduction accompanied by social mobilization activities could contribute to improved routine immunization timeliness. This study assesses the impact of Kenya's introduction of pneumococcal conjugate vaccine (PCV) on the timeliness of routine childhood vaccination in two informal, urban settlements in Nairobi. Data collected from 2007 to 2015 as part of a demographic surveillance system were used to estimate annual vaccination delays of ≥ 4 weeks among children aged 12-23 months in the period before and after the introduction of PCV in Kenya. Binomial segmented regression models using generalized estimating equations examined the association between vaccine introduction and timeliness of routine immunization. Over half of all children vaccinated in the two urban areas received one or more doses ≥ 4 weeks after the recommended age. The timeliness of routine immunization showed slight improvements or nonsignificant changes during the years following PCV introduction compared with the preceding years (adjusted prevalence ratio [aPR]: 0.67, 95% CI: 0.45-0.99 for Bacille Calmette-Guerin receipt; aPR: 0.59, 95% CI: 0.41-0.83 for third dose Pentavalent receipt; aPR: 1.19, 95% CI: 0.99-1.42 for measles). However, as of 2015, delayed vaccination remained prevalent in children, particularly among the poorest residing in the settlements. Many sub-Saharan African countries have introduced new life-saving vaccines into their routine childhood immunization schedule. Additional evidence regarding the positive or neutral influence of new vaccine introduction on the performance of delivery systems provides further justification to sustain the inclusion of these more costly vaccines in the immunization schedule.