RationaleTo prevent and control the COVID-19 pandemic, the biggest immunization campaign in history had been deployed worldwide. Therefore, it is important to inform the adverse events following immunization (AEFI) to populations.ObjectivesTo prevent vaccine hesitancy, this study focused on finding the common AEFI with the COVID-19 Comirnaty vaccine (Pfizer-BioNTech) among participants aged 18 and above and related factors in Hanoi, Vietnam.MethodsA cross-sectional study was carried out to collect participants’ data and AEFI after being vaccinated at Hanoi Medical University, Vietnam, in 2021. Logistic regression was utilized for analyzing the correlated factors of AEFI.ResultsWe recruited a random sample of 820 participants who received both basic doses of Pfizer vaccine in September and October 2021. The proportion of AEFI after the first dose, second dose, and both doses of Pfizer vaccine was 24.4%, 64.2%, and 18.5%, respectively. AEFI mostly appeared within 1 day and lasted for 1 to 2 days. The AEFI were more common in females (OR=1.7; 95%CI=1.25–2.29) and younger age groups (OR=1.9; 95%CI=1.37–2.58). History of allergy, allergic diseases, chronic diseases, and occupations were not statistically significant with AEFI.ConclusionOur findings indicated that the COVID-19 Comirnaty vaccine is safe to be injected. Gender and age group are important factors influencing AEFI.