Similarly, sex-based differences in antibody responses to vaccines have been observed in older individuals for certain vaccines (eg, influenza, zoster, and pneumococcus).19 However, no differential responses to COVID-19 were noted in our two cohorts. in Montral, Qubec, Canada. Under a rationing strategy mandated by the provincial government, adults aged 65 years and older Rabbit Polyclonal to Catenin-beta residing in long-term care facilities in Qubec, Canada, with or without previously documented SARS-CoV-2 contamination, were administered homologous or heterologous mRNA CHMFL-ABL-039 vaccines, with an extended 16-week interval between doses. All older residents in participating long-term care facilities who received two vaccine doses were eligible for inclusion in this study. Participants were enrolled from Dec 31, 2020, to Feb 16, 2021, and data were collected up to June 9, 2021. Clinical data and blood samples were serially collected from CHMFL-ABL-039 participants at the following timepoints: at baseline, before the first dose; 4 weeks after the first dose; 6C10 weeks after the first dose; 16 weeks after the first dose, up to 2 days before administration of the second dose; and 4 weeks after the second dose. Sera were tested for SARS-CoV-2-specific IgG antibodies (to the trimeric spike protein, the receptor-binding domain name [RBD] of the spike protein, and the nucleocapsid protein) by automated chemiluminescent ELISA. Two cohorts were used in this study: a discovery cohort, for which blood samples were collected before administration of the first vaccine dose and longitudinally thereafter; and a confirmatory cohort, for which blood samples were only collected from CHMFL-ABL-039 4 weeks after the primary dose. Analyses were carried out in the discovery cohort, with validation in the confirmatory cohort, when relevant. Findings The total study sample consisted of 185 participants. 65 participants received two doses of mRNA-1273 (Spikevax; Moderna), 36 received two doses of BNT162b2 (Comirnaty; PfizerCBioNTech), and 84 received mRNA-1273 followed by BNT162b2. In the discovery cohort, after a significant increase in anti-RBD and anti-spike IgG concentrations 4 weeks after the primary dose (from 486 log binding antibody models [BAU]/mL to 853 log BAU/mL for anti-RBD IgG and from 521 log BAU/mL to 805 log BAU/mL for anti-spike IgG), there was a significant decline in anti-RBD and anti-spike IgG concentrations until the boost dose (710 log BAU/mL for anti-RBD IgG and 760 log BAU/mL for anti-spike IgG), followed by an increase 4 weeks later for both vaccines (958 log BAU/mL for anti-RBD IgG and 923 log BAU/mL for anti-spike IgG). SARS-CoV-2-naive individuals showed lower antibody responses than previously infected individuals at all timepoints tested up to 16 weeks after the primary dose, but achieved comparable antibody responses to previously infected participants by 4 weeks after the second dose. Individuals primed with the BNT162b2 vaccine showed a larger decrease in mean anti-RBD and anti-spike IgG concentrations with a 16-week interval between doses (from 812 log BAU/mL to 425 log BAU/mL for anti-RBD IgG responses and from 818 log BAU/mL to 666 log BAU/mL for anti-spike IgG responses) than did those who received the mRNA-1273 vaccine (two doses of mRNA-1273: from 806 log BAU/mL to 749 log BAU/mL for anti-RBD IgG responses and from 682 log BAU/mL to 756 log BAU/mL for anti-spike IgG responses; mRNA-1273 followed by BNT162b2: from 883 log BAU/mL to 795 log BAU/mL for anti-RBD IgG responses and CHMFL-ABL-039 from 850 log BAU/mL to 797 log BAU/mL for anti-spike IgG responses). No differences in antibody responses 4 weeks after the second dose were noted between the two vaccines, in either homologous or heterologous combinations. Interpretation Interim results of this ongoing longitudinal study show that among frail, older people, previous SARS-CoV-2 contamination and the type of mRNA vaccine influenced antibody responses when used with a 16-week interval between doses. In these cohorts of frail, older individuals with a similar CHMFL-ABL-039 age and comorbidity distribution, we found that serological responses were comparable and clinically comparative between the discovery and confirmatory cohorts. Homologous and heterologous use of mRNA vaccines was not associated with significant differences in antibody responses.