Peng Y, Mentzer AJ, Liu G, et al. age groups in both cohorts. The ACE2R- blocking Abs (p<0.0001) and antibodies to the RBD of the VOCs were significantly higher in cohort 2, compared to cohort 1. 41.2% to 65.8% of different age groups gave a positive response by the haemagglutination assay to the RBD of the ancestral virus and VOCs in cohort 1, while 53.6% Basmisanil to 90% gave a positive response in cohort 2. 17/57 (29.8%) of cohort 1 and 17/29 (58.6%) of cohort 2 had ex vivo IFN ELISpot responses above the positive threshold. The ACE2R-blocking antibodies and ex vivo IFN ELISpot responses at 12 weeks post-first dose, significantly correlated with levels 12 weeks post second dose (Spearmans r=0.46, p=0.008) and (Spearmans r=0.71, p<0.0001) respectively. Conclusions: Both dosing schedules resulted in high levels of antibody and T cell responses post vaccination, although those with a longer dosing gap had a higher magnitude of responses, possibly as immune responses were measured 6 weeks post second dose compared to 12 weeks post second dose. Background The ChAdOx1 nCoV-19 vaccine (AZD1222) vaccine, is a non-replicating chimpanzee adenovirus vector vaccine, containing the sequence for the spike protein of the ancestral SARS-CoV-2 virus1. Although the vaccine was initially administered with a dosing gap of 4 weeks between the two doses, subsequently the dosing gap was increased to 12 weeks, as it was shown to increase the efficacy of the vaccine2. However, many countries increased the dosing gap to 16 weeks in order to administer a single dose to a larger population3 4 and also due to the delay in obtaining adequate vaccines for administering the second dose on time5. It was later shown that an increase in the gap between the two doses beyond 12 weeks, and even up to 45 weeks, resulted in higher antibody titres after the second dose of the vaccine6. AZD1222 was the first vaccine to be rolled out in Sri Lanka, with immunization of the health care workers. However, after it was rolled out to the public, due to the delay in obtaining the second Basmisanil dose, most individuals received their second dose 20 weeks after obtaining their first dose. We showed that at 16 weeks post-immunization Basmisanil with a single dose of AZD1222, 93.7% of those >70 years had SARS-CoV-2 specific antibodies, although ACE2 receptor blocking antibodies (those that correlate with neutralizing antibodies) were significantly less in those >60 years of age5. However, robust memory T cell and B cell responses were seen in over 90% of individuals. Although it has been shown that an increase in the gap between the two doses subsequently led to higher antibody titres6, there are limited data in the differences in dosing gaps on antibody responses to SARS-CoV-2 variants of concern (VOCs), differences in antibody responses in different age groups, those with comorbidities and also the influence of the dosing gap on T cell responses. Currently many studies have shown that there is waning of immunity with many of the COVID-19 vaccines following the second dose7C9. Due to an increased rate of breakthrough infections, some which led to hospitalization and death, due to waning of immunity, a booster dose is recommended to elderly individuals and the immunocompromised by many authorities10 11. While waning of antibody levels alone does not necessarily imply waning Rabbit Polyclonal to OR10C1 of efficacy12 and protection, it is important to find out if different dosing schedules lead to differences in the quality and quantity of immune responses and thus, an impact of the duration of immunity. In order to answer these questions, we compared the immune responses of two cohorts of Sri Lankan individuals who received the AZD1222 vaccine at 12-week dosing intervals.