Outcome after single dose of ChAdOx1 vaccine against SARS-CoV-2 infection at 16 weeks post-vaccination among healthy adults in Saudi Arabia

BACKGROUND: The rate of severe acute respiratory syndrome coronavirus 2 (SARS CoV-2) infection and immunogenicity of a single dose of ChAdOx1 vaccine at 16 weeks post-vaccination among young and healthy participants remains unclear in Saudi Arabia. OBJECTIVES: Assess the rate of subsequent infection and immunogenicity of a single dose of ChAdOx1 vaccine at 16 weeks post-vaccination in a sample of healthy and young participants. DESIGN: Cross-sectional study SETTING: Academic teaching hospital in Riyadh, Saudi Arabia SUBJECTS AND METHODS: Healthy participants 18–50 years of age, who received one dose of ChAdOx1 vaccine and had no history of SARS CoV-2 infection were recruited, and blood samples were obtained 16 weeks after vaccination to assess immunogenicity using a commercially available kit. MAIN OUTCOME MEASURES: The rate of SARS-CoV-2 infection within 16 weeks post-vaccination. SAMPLE SIZE: 385 participants with median (IQR) age of 34 (29-38) years. RESULTS: Eleven (2.8%) participants acquired polymerase chain reaction (PCR)-confirmed infection within 16 weeks after a single dose of ChAdOx1 vaccine (mean [SD] 42.5 [28] days post-vaccination). No hospital or intensive care unit admissions occurred among the subjects in this sample. Females were significantly over-represented in PCR-confirmed cases of SARS-CoV-2 infection, with 10 of 11 infections occurring in females (P=.006). Antibody response against anti-spike IgG were detectable in 92.7% of subjects at 16 weeks’ post-vaccination. The median anti-spike IgG level after vaccination was 273.1 (IQR 107-1052 AU/mL). However, the anti-nucleocapsid IgG antibody demonstrated a sensitivity of only 20%. CONCLUSION: A single dose of ChAdOx1 vaccine in healthy and young individuals was associated with a low, single-digit rate of PCR-confirmed infection, most of which were mild. LIMITATIONS: Small sample size and single-center. CONFLICT OF INTEREST: None.