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Purpose This study was conducted to investigate antibody immune responses induced by BNT162b2 and AZD1222 human COVID-19 vaccines in Riyadh city, Saudi Arabia. Patients and Methods ELISA was used to evaluate antibodies, against the SARS-CoV-2 spike S1 protein, in serum samples from 432 vaccinated individuals at six time points: pre-vaccination (baseline), post-prime, post-boost, 6-months, and 1 year post-vaccination, and 3 weeks post a third dose. Virus microneutralization assay was used to confirm antibody responses in a subset of samples. Results Anti-SARS-CoV-2 spike IgG were detected in most subjects post-prime, reached a peak level post-boost, and remained at high level at the 6-month follow-up. At 1 year post-vaccine, the antibody levels were low but increased to a significant level higher than the peak following a third dose. The third dose was given at an average of 250 days after the second dose. The virus microneutralization assay confirmed the neutralization activity of the induced SARS-CoV-2 IgG antibodies. The vaccines induced higher IgG titres at post-prime ( p =0.0001) and 6 months ( p =0.006) in previously infected individuals. An increased interval between prime and boost, more than recommended time, appeared to enhance the IgG levels ( p =0004). Moreover, the vaccines induced higher IgG levels in younger subjects ( p =0.01). Conclusion These data provide insights and build on the current understanding of immune responses induced by these two vaccines; and support a third boosting dose for these COVID-19 vaccines.

Objectives: To 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). Methods: A 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. Results: Subjects 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. Conclusion: Single-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.