Title : Real-world COVID-19 vaccine effectiveness in Ethiopia using a retrospective test-negative case-control study design
Abstract:
Background: Coronavirus Disease (COVID-19) is a global health emergency caused by SARS-CoV-2. Vaccination is a key prevention and control strategy. Ethiopia began COVID-19 vaccination in March 2021, but data on vaccine effectiveness remain limited, particularly in African countries. This study aimed to evaluate the real-world effectiveness of COVID-19 vaccines in Ethiopia.
Method: A retrospective test-negative case-control study was conducted using 2,000 participants (1:1 case-to-control ratio), selected from national vaccine and DHIS-2 databases through probability sampling. COVID-19-positive cases were matched with test-negative controls by age, sex, testing date, and SARS-CoV-2 transmission season. To minimize the impact of variant shifts, data were extracted during four post-vaccination surge periods: T1 (Mar-May 2021), T2 (Aug-Oct 2021), T3 (Dec 2021-Jan 2022), and T4 (May-July2022). Missing information was collected via phone interviews after obtaining informed consent. Conditional logistic regression models estimated the risk of testing positive for SARS-CoV-2, with vaccination status as the primary exposure. Odds ratios (ORs) and 95% confidence intervals (CIs) were calculated for infection, hospitalization, and death. Vaccine effectiveness (VE) was estimated as VE = (1 – OR) × 100. Stratified analyses were performed by age group, chronic illness, and calendar time.
Results: Partial vaccination reduced the risk of testing positive by 16.7% (95% CI: –38.8%, 50%), while full vaccination had an effectiveness of 24.6% (95% CI: –63.5% to 5.1%). Severity of COVID-19 was significantly associated with having a chronic illness (AOR = 4.14; 95% CI: 2.4, 7.02), unawareness of testing positive (AOR = 42.08; 95% CI: 1.95, 909.69), and current alcohol use (AOR = 1.95; 95% CI: 1.02, 3.70).
Conclusion: COVID-19 vaccines in Ethiopia showed limited protection against infection, possibly due to emerging variants, but appeared to reduce disease severity. Chronic illness, lack of test awareness, and alcohol use were associated with severe outcomes, highlighting the need for targeted interventions.
