question: ObjectivesTo describe the real-world data on the effectiveness of Pfizer-BioNtech BNT162b2 and AstraZeneca-Oxford AZD1222 vaccines against COVID-19 in a large cohort in the Kingdom of Saudi Arabia (KSA). MethodsA total of 18,543 subjects received a single-dose of either of the vaccines at one vaccination centre in KSA, and were followed up for three to eight months. Clinical data from medical records, adverse events (AEs) from a self-reporting system, and COVID-19 infection data from the national databases were retrieved and analysed. ResultsSubjects median age was 33 years old with an average of 27.3 body mass index and the majority were male (60.1%). 92.17% of the subjects had no COVID-19 infection post-vaccination. Diabetes mellitus (p=0.0325), organ transplantation (p=0.0254), and morbid obesity (p=0.0014) were risk factors for infection post-vaccination. Unlike vaccine type, being Saudi, male, or obese was more likely to get the infection earlier. AE reports from 1084 subjects included injection site pain, fatigue, fever, myalgia, headache. ConclusionSingle-dose COVID-19 vaccines in KSA showed an effectiveness rate of 92.17% up to eight months follow-up. The rate for AZD1222 was higher than what have been previously reported. Side effects and AEs were within what has been reported in clinical trials.