(A-346) Background Rates of Adverse Events for Vaccine Safety Monitoring at Two Sentinel Hospitals in Addis Ababa, Ethiopia

Background: Background rates of adverse events are important for monitoring and interpreting the safety of vaccines, especially during mass vaccination campaigns. The lack of harmonized case validation in real-world data creates heterogeneity across background rates for adverse events of special interest (AESI) and makes it challenging to assess safety signals. Also, background rates are often unavailable in low- and middle-income countries.

Objectives: To generate background rates of selected AESIs in Addis Ababa, Ethiopia.

Methods: Active surveillance was conducted at a primary (Hospital A) and a referral (Hospital B) public hospital from November 2023 to October 2024. All patients presenting with one or more of the 11 pre-specified AESIs at the emergency outpatient departments or inpatient wards were identified prospectively. Trained physicians used structured data collection forms with algorithms to capture patient information and validated the AESI diagnoses against the Brighton Collaboration case definition level of diagnostic certainty. The background rates of the top four confirmed (levels of diagnostic certainty 1–3) AESIs (thrombosis/thromboembolism, generalized convulsion, thrombocytopenia, and Guillain-Barré syndrome [GBS]) were calculated overall, and for age- and sex-strata per 100,000 encounters.

Results: We identified 232 patients with confirmed AESIs at Hospital A and 139 at Hospital B. The most common AESIs per 100,000 at Hospital A were thrombosis/thromboembolism (592.0, 95% CI 488.5–695.5), generalized convulsion (412.0, 95% CI 325.6–498.4), GBS (37.9, 95% CI 11.6–64.1), and thrombocytopenia (33.2, 95% CI 8.6–57.7); and at Hospital B, were thrombosis/thromboembolism (429.5, 95% CI 329.8–529.2), generalized convulsion (284.3, 95% CI 203.1–365.5), thrombocytopenia (102.8, 95% CI 54.0–151.7), and GBS (24.2, 95% CI 0.5–47.9). The background rates also varied by age and sex. At both hospitals, the rate of thrombosis/thromboembolism was higher in females compared to males, while generalized convulsion was higher in males. A higher rate of generalized convulsion was observed in age group 0–17, while thrombosis/thromboembolism was higher in age groups 45–64 and 65+. Thrombocytopenia was detected only in age groups 0–17 and 18–44.

Conclusions: We established background rates through standardized methodology and validation of case definitions at sentinel hospitals. These background rate data can be used to facilitate detection of potential safety signals following vaccination in Ethiopia.