Implementation and adherence of routine pertussis vaccination (DTP) in a low-resource urban birth cohort

Introduction: Reliable information on rates of up-to-date coverage and
timely administration of routine childhood immunizations is critical for
guiding public health efforts worldwide, yet prospective observation of
vaccination programs within individual communities is rare. Here we
provide a longitudinal analysis of the directly-observed administration of
a 3-dose primary vaccination series to infants in a low-resource community
in Lusaka, Zambia. Methods: Throughout 2015, we recruited a longitudinal
birth cohort of mother/infant pairs (initial enrollment, 1,981 pairs;
attending, 1,497 pairs) from the peri-urban informal settlement of Chawama
compound, located in Lusaka, Zambia. We prospectively monitored the
administration of scheduled Diphtheria-Tetanus-Pertussis (DTP)
vaccinations across the first 14-18 weeks of life. We analyzed study
attendance and vaccine coverage, both overall and stratified by age group.
We employed Kaplan-Meier analyses to estimate delays in age-appropriate
administration of vaccine doses. We also assessed schedule timing
violations, including early and compressed dose administration. Results:
At study completion, first dose (DTP1) rates were high (92.9% of
attending), whereas third dose completion (DTP3) rates were far lower
(61.9%). Missed vaccinations and study dropout both contributed to the low
DTP3 completion rates. DTP1 was administered very late (at or after 10
weeks) to 61 infants (4.1%). DPT1 was administered too early to 64 infants
(4.3%), and 77 (5.1%) received consecutive doses below the minimum
recommended spacing of 28 days. Conclusions: We observe substantial
individual variation in the timing of early childhood DTP doses, though
following this birth cohort proved challenging. Our results indicate that
timely administration of both DTP1 and DPT3 remains a challenge in this
community. These directly-observed, individual-based results provide an
important counterpoint to more course-grained, survey-based national and
province estimates of up-to-date vaccine coverage. This study also
highlights the challenges of vaccine hesitancy and sub-optimal utilization
of (no-cost) healthcare services in a low-resource urban setting.