Malaria vaccine coverage estimation using age-eligible populations and service user denominators in Kenya

Abstract

Background: The World Health Organization approved the RTS,S/AS01 malaria vaccine for wider rollout, and Kenya participated in a phased pilot implementation from 2019 to understand its impact under routine conditions. Vaccine delivery requires coverage measures at national and sub-national levels to evaluate progress over time. This study aimed to estimate the coverage of the RTS,S/AS01 vaccine during the first 36 months of the Kenyan pilot implementation.

Methods: Monthly dose-specific immunization data for 23 sub-counties were obtained from routine health information systems at the facility level for 2019-2022. Coverage of each RTS,S/AS01 dose was determined using reported doses as a numerator and service-based (Penta 1 and Measles) or population (projected infant populations from WorldPop) as denominators. Descriptive statistics of vaccine delivery, dropout rates and coverage estimates were computed across the 36-month implementation period.

Results: Over 36 months, 818,648 RTSS/AS01 doses were administered. Facilities managed by the Ministry of Health and faith-based organizations accounted for over 88% of all vaccines delivered. Overall, service-based malaria vaccine coverage was 96%, 87%, 78%, and 39% for doses 1-4 respectively. Using a population-derived denominator for age-eligible children, vaccine coverage was 78%, 68%, 57%, and 24% for doses 1-4, respectively. Of the children that received measles dose 1 vaccines delivered at 9 months (coverage: 95%), 82% received RTSS/AS01 dose 3, only 66% of children who received measles dose 2 at 18 months (coverage: 59%) also received dose 4.

Conclusion: The implementation programme successfully maintained high levels of coverage for the first three doses of RTSS/AS01 among children defined as EPI service users up to 9 months of age but had much lower coverage within the community with up to 1 in 5 children not receiving the vaccine. Consistent with vaccines delivered over the age of 1 year, coverage of the fourth malaria dose was low. Vaccine uptake, service access and dropout rates for malaria vaccines require constant monitoring and intervention to ensure maximum protection is conferred.

Keywords: Kenya; Malaria; Malaria vaccine pilot; RTS,S/AS01; Vaccine coverage.

Fig. 1

Geographic distribution of vaccinating health facilities across RTS,S/AS01 vaccine implementation sub-counties in Western Kenya (N = 537). Primary care facilities are classified as Level 2–3 and include clinics, dispensaries and health centres while secondary care facilities are classified as Level 4–5 and comprise primary and secondary referral hospitals [25]

Fig. 2

Total RTS,S/AS01 vaccine doses administered over 36 months by each health facility (N = 537). High volume (> 3000 doses) facilities: ¹Jaramogi Oginga Odinga Teaching & Referral Hospital, ²Nightingale Medical Centre, ³Lumumba Sub County Hospital, ⁴Malava Sub County Hospital, ⁵Kimaeti Health Centre, ⁶Nambale Sub County Hospital

Fig. 3

Total monthly doses of RTS, S/AS01 vaccine reported to DHIS2 (September 2019 to August 2022). ¹First case of COVID-19 reported in Kenya,²National health worker strike (Dec 2020–Feb 2021)

Fig. 4

Reported cumulative doses of RTS,S/AS01 vaccines administered over 36 months relative to key infant vaccinations. ¹First case of COVID-19 reported in Kenya, ²National health worker strike (Dec 2020–Feb 2021), ³Total Population under 1, ⁴EPI population (Penta 1), ⁵EPI population (MR1-9 months), ⁶EPI population (MR2-18 months), ⁷Total Population over 1 expected to turn 2 within the year