ABSTRACT
Background: In attempts to rapidly immunize a greater proportion of the Ontario population against COVID, public health officials recommended extending the interval between vaccine doses and allowed "mixing of vaccine types". The impact of these decisions on the antibody response to the vaccine, particularly in the community dwelling elderly population is unknown. Methods: The STOPCoV study is designed to compare the IgG antibody response to spike protein and receptor binding domain (RBD) after COVID vaccination in those aged ≥ 70 years relative to a cohort aged 30-50 years. This prospective decentralized observational study is conducted remotely on a digital platform (www.stopcov.ca). Participants signed an e-consent, completed questionnaires and will submit dried blood spot (DBS) specimens 6-8 times over 48 weeks after the second vaccine dose. DBS samples were analyzed for IgG antibodies to spike and RBD by an in-house ELISA. We report here the ratio-normalized levels of anti-spike and anti-RBD IgG antibodies prior to and at 2 weeks after the second vaccine with comparisons between age groups. Linear regression models were used to determine the effect of age on the ratio normalized RBD antibody levels 2 weeks post second dose of vaccine after adjusting for potential confounders determined a priori. Results: 1286 persons enrolled between May 17 and July 31, 2021. 1194 participants (853 > 70 years;341 aged 30-50) completed at least one study related task. 761 (64.1%) are female. Most received an mRNA vaccine, with 863 (74%) receiving the same vaccine brand, and 196 (17%) receiving mixed brands over 2 doses. Two weeks after the second vaccine dose, the median interquartile rangeanti-spike antibody level was 0.76 [0.45, 1.16] for those ≥70 compared to 1.3 [0.98, 1.56] for those 30-50 (p<0.001). The median anti-RBD antibody levels were 0.28 [0.15, 0.53] and 0.66 [0.41, 1.08] (p<0.001) for the older and younger cohorts respectively. After adjusting for gender, cardiovascular disease, cancer, diabetes, transplant or immune suppression, body mass index, vaccine brand, and time between doses, participants ≥70 had lower levels of anti-RBD antibodies at 2 weeks after 2nd dose (β=-0.14, 95% confidence interval-0.19,-0.08, p<0.0001). Conclusion: High antibody levels against COVID-19 are attainable after 2 doses of mRNA vaccines. Levels were higher with Moderna than Pfizer. Delay of the second dose to 4 months or mixing of brands had minimal impact on the antibody level but levels are lower in the elderly.