Title : Real-world COVID-19 vaccine effectiveness in Ethiopia using a retrospective test-negative case-control study design
Abstract:
Background: The Coronavirus Disease 2019 (COVID-19) pandemic has been a global health emergency caused by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2). COVID-19 vaccinations are among the significant prevention and control mechanisms. Ethiopia launched vaccination in March 2021. However, there is a paucity of published data on the effectiveness of COVID-19 vaccines like in many African countries. Therefore, this study aimed at evaluating the real world COVID-19 vaccine effectiveness in Ethiopia.
Method: In this retrospective test-negative case-control study, we selected 2000 study participants with 1:1 ratio of cases and controls from the national vaccine and DHIS-2 databases using probability sampling technique. COVID-19-positive cases were randomly matched to test-negative controls based on age, sex, testing date and SARS-CoV-2 transmission peak season in a 1:1 ratio. To avoid the impact of circulating variant shift over period, the data extraction periods were matched to the occurrence of the COVID-19 surges, which were four different time points (T1 = March-May 2021; T2 = August-October 2021; T3 = December 2021-January 2022 and T4 = May-July 2022) in which post-COVID-19 vaccination rollout COVID-19 surges have occurred in Ethiopia. Information which was not available in the data bases were collected through telephone interview after an informed consent is obtained. Conditional logistic regression models were used to estimate the risk of testing positive for SARS CoV-2 with the PCR with vaccination status as the primary exposure of interest. The odds of having COVID-19 was estimated among vaccinated and unvaccinated persons, and the corresponding odds ratio and accompanying 95% confidence intervals. Odds ratios for COVID-19-related death and hospitalization was calculated for cases and controls and used to estimate vaccine effectiveness. Vaccine effectiveness then estimated as one minus the odds ratio (OR), VE= (1-OR) *100. A series of stratified analyses were done for VE according to age group, presence of at least one chronic condition, and calendar time.
Result: Partial COVID-19 vaccination reduces the risk of testing positive by 16.7% (95%CI: -38.8%, 50%) compared to those who were not vaccinated. In contrast, the effectiveness of full vaccination was 24.6% (95%CI: -63.5% to 5.1%). The severity of COVID-19 was associated with having chronic illness (AOR=4.14; 95%CI: 2.4, 7.02), not knowing that they were tested positive for SARS-CoV-2 (AOR=42.08; 95%CI: 1.95, 909.69) and currently drinking alcohol (AOR=1.95; 95%CI: 1.02, 3.70).
Conclusion: This study shows that the efficacy of COVID-19 vaccination in Ethiopia, whether partial or full doses, provides limited protection, possibly related to the continued emergence of new variants during the pandemic period, but reduces disease severity. In this study, the presence of chronic illness, knowledge of being tested positive, and current alcohol drinking were associated with the severity of COVID-19, suggesting the importance of targeted preventive interventions for individuals at high risk of developing severe COVID-19.
Key words: COVID-19, Ethiopia, SARS-CoV-2, Vaccine Effectiveness.
