Safety and Immunogenicity of a Modified COVID-19 MRNA Vaccine, SYS6006, As a Fourth-dose Booster Following Three Doses of Inactivated Vaccines in Healthy Adults: an Open-labeled Phase 1 Trial

The continuous emergence of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants led to a rapid decline in protection efficacy and neutralizing titers even after three doses of COVID-19 vaccines. Here, we report an open-labeled Phase I clinical trial of a modified mRNA vaccine (SYS6006) as a fourth-dose booster in healthy adults. Eighteen eligible participants, who had completed three doses of inactivated COVID-19 vaccines, received a fourth boosting dose of SYS6006-20 μg. Eighteen convalescent COVID-19 patients were enrolled for the collection of serum samples as a comparator of immunogenicity. The primary endpoint of this trial was titers of anti-receptor binding domain of spike glycoprotein (RBD) antibodies of the Omicron strain (BA.2 and BA.4/5) in serum; titers of neutralizing antibodies against pseudovirus of the Omicron strain (BA.2 and BA.4/5). The secondary endpoint was the incidence of adverse events within 30 days after the boosting. The exploratory endpoint was the cellular immune responses (interferon gamma, IFN-γ). This trial was registered with the Chinese Clinical Trial Registry website. No serious adverse events were reported within 30 days after vaccination. No Grade 3 fever or serious adverse event was reported in the SYS6006 group. Notably, SYS6006 elicited higher titers and longer increases in anti-RBD antibodies and neutralizing antibodies (>90 days) compared with the convalescent group ( < 0.0001) against Omicron strain (BA.2 and BA.4/5). Besides, higher positive spots of T-cell-secreting IFN-γ were observed in the SYS6006 group than those in the convalescent group ( < 0.05). These data demonstrated that SYS6006 was well tolerated and highly immunogenic, generating a stronger and more durable immune response against different variants of SARS-CoV-2.