Feasibility of Cohort Event Monitoring and Assessment of Reactogenicity and Adverse Events Among a Cohort of AstraZeneca and Moderna COVID-19 Vaccine Recipients in Nigeria, 2021

Background: To generate COVID-19 vaccine safety data in Nigeria, passive reporting was supplemented with cohort event monitoring (CEM), an active surveillance system. We described reactogenicity within 7 days and adverse events up to 3 months after each AstraZeneca or Moderna COVID-19 vaccine dose while assessing the feasibility of implementing CEM in a low- to middle-income country (LMIC) during a mass vaccination campaign.

Methods: Participants were aged ≥18 years with access to mobile phones who received the first dose of an authorized COVID-19 vaccine from participating health facilities in 6 states of Nigeria during September and October 2021. Data collectors interviewed participants via phone on days 0, 3, 7, and thereafter every 7 days for 3 months. The same schedule was restarted if a participant received a second vaccine dose. Proportions of participant-reported adverse events following COVID-19 vaccine receipt were calculated. Investigation and causality assessment were conducted on deaths using the World Health Organization causality guidelines.

Results: We enrolled 12,317 participants (AstraZeneca 6990; Moderna 5327); 6167/6990 (88.2 %) AstraZeneca and 4879/5327 (91.6 %) Moderna recipients completed a follow-up interview days 0-7 after the first dose; among them, 2685/6167 (43.5 %) AstraZeneca and 3533/4879 (72.4 %) Moderna recipients reported local reactions and 2456/6167 (39.8 %) AstraZeneca and 2087/4879 (42.8 %) Moderna recipients reported systemic reactions. Overall, 3891/6990 (55.7 %) AstraZeneca and 3978/5327 (72.8 %) Moderna recipients received a second dose of COVID-19 vaccine, among whom 897/3891 (23 %) AstraZeneca and 1979/3978 (49.7 %) Moderna recipients reported local reactions and 727/3891 (18.7 %) AstraZeneca and 1680/3978 (42.2 %) Moderna recipients reported systemic reactions. Among all enrolled, 11 died; there was no evidence to suggest any deaths were vaccine-related.

Conclusions: No unexpected patterns of adverse events were detected, providing additional data on the safety of these COVID-19 vaccines in Nigerian adults. We demonstrated that implementing CEM was feasible and may be valuable for safety monitoring of vaccines introduced in LMICs.