Mucosal and Systemic Antibody Responses after Boosting with a Bivalent Messenger RNA Severe Acute Respiratory Syndrome Coronavirus 2 Vaccine

Background Mucosal immunity plays a critical role in preventing severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and replication. Understanding the capacity of coronavirus disease 2019 (COVID-19) vaccines to elicit both mucosal and systemic antibodies could help optimize vaccination strategies. Methods We conducted an open-label, phase 1/2 adaptive-design clinical trial to evaluate the safety and immunogenicity of COVID-19 immunizations. Healthy adults received 2 priming doses of mRNA-1273, a booster dose of mRNA-1273, and a second booster of bivalent (WA-1 and BA.4/BA.5) mRNA-1273.222. Adverse event data were collected. Serum and mucosal immunity were evaluated. Results One hundred six persons were enrolled. Thirty received all 4 study-related vaccine doses. All vaccines were well tolerated, with injection site pain, malaise, myalgias, and headache being the most frequently reported symptoms. Among those who received a second booster, 24 of 30 (80%) had serological evidence of SARS-CoV-2 infection. Following the second booster, increases in geometric mean binding and pseudovirus neutralization antibody titers to the ancestral strain and BA.1 and BA.5 variants were observed. Increases in mucosal immunoglobulin G and immunoglobulin A (IgA) antibodies in nasal and salivary samples were observed in both previously infected and infection-naive participants, although prior infection markedly boosted virus-specific mucosal IgA responses. Conclusions The mRNA-1273.222 booster vaccine was safe and immunogenic and induced mucosal antibody responses in previously infected and infection-naive persons.