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Background: Current vaccines against COVID‐19 effectively reduce morbidity and mortality and are vitally important for controlling the pandemic. Between December 2020 and February 2021, adenoviral vector vaccines such as ChAdOx1 (AstraZeneca‐Oxford) were put in use. Recent reports demonstrate robust serological responses to a single dose of messenger RNA vaccines in individuals previously infected with SARS‐CoV‐2. We aimed to study the association between previous COVID‐19 infection and antibody levels after a single dose of ChAdOx1 nCoV‐19. Methods: This cross‐sectional study was conducted on 657 individuals who were either convalescent or SARS‐CoV‐2 naive and had received one dose of ChAdOx1 (AstraZeneca). A questionnaire was used to collect data on age, sex, and self‐reported history of COVID‐19 infection. We then compared the average levels of immunoglobulin G (IgG) between the previously infected and COVID‐19‐naive participants. Results: We compared the antibody responses of individuals with confirmed prior COVID‐19 infection with those of individuals without prior evidence of infection. The mean antibody levels in those who reported no history of COVID‐19 infection were substantially lower than in those who were previously infected, in both males and females. Sex‐related differences were observed when we compared antibody levels between men and women. In males, anti‐S IgG antibody levels were higher in those who had been previously infected (156.1 vs. 87.69 AU/mL, p =.009), compared with the same pattern was observed in females (113.5 vs. 90.69 AU/mL, p =.005). Conclusions: Previous COVID‐19 infection is associated with higher levels of SARS‐CoV‐2 antibodies following ChAdOx1 (AstraZeneca) vaccination. Our finding supports the notion that a single dose of ChAdOx1 nCoV‐19 administered post‐SARS‐CoV‐2 infection serves as an effective immune booster. This provides a possible rationale for a single‐dose vaccine regimen for previously infected individuals. [ABSTRACT FROM AUTHOR]

Measurement of strength and durability of SARS-COV-2 antibody response is important to understand the waning dynamics of immune response to both vaccines and infection. The study aimed to evaluate the level of IgG antibodies against SARS-CoV-2 and their persistence in recovered, naïve, and vaccinated individuals. We investigated anti-spike RBD IgG antibody responses in 10,000 individuals, both following infection with SARS-CoV-2 and immunization with SARS-COV-2 AstraZeneca, Sputnik V, Sinopharm, and Sinovac. The mean levels of anti-spike IgG antibodies were higher in vaccinated participants with prior COVID-19 than in individuals without prior COVID-19. Overall, antibody titers in recovered vaccinee and naïve vaccinee persisted beyond 20 weeks. Vaccination with adenoviral–vector vaccines (AstraZeneca and Sputnik V) generates higher antibody titers than with killed virus vaccine (Sinopharm and Sinovac). Approximately two-thirds of asymptomatic unvaccinated individuals had developed virus-specific antibodies. A single dose of vaccine is likely to provide greater protection against SARS-CoV-2 infection in individuals with apparent prior SARS-CoV-2 infection, than in SARS-CoV-2-naive individuals. In addition, the high number of seropositivity among asymptomatic unvaccinated individuals showed that the number of infections are probably highly underestimated. Those vaccinated with inactivated vaccine may require more frequent boosters than those vaccinated with adenoviral vaccine. These findings are important for formulating public health vaccination strategies during COVID-19 pandemic. [ABSTRACT FROM AUTHOR]