Objectives: The aim of this study was to assess the safety and immunogenicity of a dose-sparing fractional intradermal (ID) booster strategy with the mRNA-1273 COVID-19 vaccine. Methods: COVID-19... Show moreObjectives: The aim of this study was to assess the safety and immunogenicity of a dose-sparing fractional intradermal (ID) booster strategy with the mRNA-1273 COVID-19 vaccine. Methods: COVID-19 naive adults aged 18e30 years were recruited from a previous study on primary vaccination regimens that compared 20 mg ID vaccinations with 100 mg intramuscular (IM) vaccinations with mRNA-1273 as the primary vaccination series. Participants previously immunized with ID regimens were randomly assigned (1:1) to receive a fractional ID booster dose (20 mg) or the standard-of-care intramuscular (IM) booster dose (50 mg) of the mRNA-1273 vaccine, 6 months after completing their primary series (ID-ID and ID-IM group, respectively). Participants that had received a full dose IM regimen as the primary series, received the IM standard-of-care booster dose (IM-IM group). In addition, COVID-19 naive individuals aged 18e40 years who had received an IM mRNA vaccine as the primary series were recruited from the general population to receive a fractional ID booster dose (IM-ID group). Immunogenicity was assessed using IgG anti-spike antibody responses and neutralizing capacity against SARS-CoV-2. Cellular immune responses were measured in a sub-group. Safety and tolerability were monitored. Results: In January 2022, 129 participants were included in the study. Fractional ID boosting was safe and well tolerated, with fewer systemic adverse events compared with IM boosting. At day 28 post-booster, anti-spike S1 IgG geometric mean concentrations were 9106 (95% CI, 7150e11 597) binding antibody units (BAU)/mL in the IM-IM group and 4357 (3003e6322) BAU/mL; 6629 (4913e8946) BAU/mL; and 5264 (4032e6873) BAU/mL in the ID-IM, ID-ID, and IM-ID groups, respectively. Discussion: Intradermal boosting provides robust immune responses and is a viable dose-sparing strategy for mRNA COVID-19 vaccines. The favourable side-effect profile supports its potential to reduce vaccine hesitancy. Fractional dosing strategies should be considered early in the clinical development of future mRNA vaccines to enhance vaccine availability and pandemic preparedness. Show less
