The reactogenicity and immunogenicity of a booster dose after the second dose of a protein subunit vaccine MVC-COV1901: An extension to an open-label, dose-escalation, and non-randomized phase 1 study

The waning antibody levels after immunization and the emergence of SARS-CoV-2 variants of concern (VoC) negatively impact the vaccine-induced neutralization of SARS-CoV-2. In this extension to the phase 1 clinical study, we report the antibody durability until 180 days after the second dose of MVC-COV1901, and examined the reactogenicity and immunogenicity followed by a booster shot MVC-COV1901 administered to 45 healthy adults from 20 to 49 years of age on day 209. Adverse reactions after the booster dose were mostly mild and comparable to that of the first two doses. Neutralizing antibodies remained detectable on day 209 at 59.4, 79.4, and 113.2 (IU/mL) for low dose (LD), middle dose (MD), and high dose (HD) groups, respectively. At four weeks after the booster dose, neutralizing titers increased to 1719.6, 818.3, and 1345.6 for LD, MD, and HD groups, respectively. Our data also showed that three doses of MVC-COV1901-induced antibodies were still effective, albeit lowered neutralizing titers, against the Omicron variant.