Abstract
BACKGROUND: Given the limited evidence on the side effects of AZD1222 booster in individuals previously vaccinated with Sinopharm BBIBP-CorV, we conducted a cohort event monitoring study to capture adverse events in those receiving AZD1222 booster doses (first or second) after Sinopharm BBIBP-CorV vaccination in Iran. METHODS: This active COVID-19 vaccine safety surveillance study was conducted in vaccination centers across 12 provinces in Iran. The study population included individuals who had received two doses of Sinopharm BBIBP-CorV and either a first or second AZD1222 booster dose. Each participant was followed for 13 weeks after their booster dose. RESULTS: A total of 28,742 participants were included, with 13,428 females (46.72%) and 15,314 males (53.28%). The average age was 43.25 ± 14.90 years. Of the total, 82.46% were recruited retrospectively through vaccination registration. Among 16,944 individuals with available polymerase chain reaction test results, 118 tested positive for COVID-19 between 14 and 90 days post-booster. There were 123 hospital admissions, with 18 cases showing a higher likelihood of being associated with adverse events following immunization (AEFIs) or serious adverse events (SAEs). After excluding deaths unrelated to vaccination, four deaths were potentially linked to AEFIs or SAEs. The cumulative incidence rates for hospital admissions and deaths were 106.40 and 13.90 per 100,000 individuals, respectively. CONCLUSION: The study indicates that AZD1222 booster doses are safe for individuals previously vaccinated with Sinopharm BBIBP-CorV, with rare adverse events and an acceptable safety profile. This supports the use of heterologous booster strategies, though ongoing monitoring and improved pharmacovigilance are essential to detect any potential long-term effects.