Trial Design and Participants
We initially conducted a phase 1, dose-escalation, open-label clinical trial of mRNA-1273 involving participants between the ages of 18 and 55 years2 in which we evaluated doses of 25 μg, 100 μg, and 250 μg. We subsequently expanded the trial to include 40 participants who were 56 years of age or older and who were stratified into two subgroups: those between the ages of 56 and 70 years and those who were 71 years of age or older. Because of clinically significant systemic reactogenicity observed in participants between the ages of 18 and 55 years at the 250-μg dose, we administered doses of 25 μg or 100 μg to the older participants.
The trial was conducted at Kaiser Permanente Washington Health Research Institute in Seattle, the Emory University School of Medicine in Atlanta, and the National Institute of Allergy and Infectious Diseases (NIAID) Vaccine Research Center in Bethesda, Maryland. Enrolled adults were healthy and provided written informed consent before undergoing any study procedures. We did not screen for evidence of past or current SARS-CoV-2 infection by testing blood or nasal specimens before enrollment. Full eligibility criteria, along with details of the trial design, conduct, oversight, and statistical analyses, are described in the protocol, which is available with the full text of this article at NEJM.org.
The mRNA-1273 vaccine was codeveloped by researchers at the NIAID Vaccine Research Center and Moderna in Cambridge, Massachusetts. This vaccine encodes a stabilized version of the SARS-CoV-2 full-length spike glycoprotein trimer, S-2P, which has been modified to include two proline substitutions at the top of the central helix in the S2 subunit. The mRNA is encapsulated in lipid nanoparticles at a concentration of 0.5 mg per milliliter and diluted with normal saline to achieve the final target vaccine concentrations.
The NIAID served as the trial sponsor and made all decisions regarding the study design and implementation. The vaccine Investigational New Drug application and the protocol amendment expanding the age subgroups were reviewed by the Food and Drug Administration and the institutional review board at Advarra, a regulatory compliance consulting company, which served as the single institutional review board for all the study sites. An independent data and safety monitoring committee reviewed interim safety reports.
Moderna provided mRNA-1273 for use in this trial but did not provide any financial support. Employees of Moderna collaborated on the development of the protocol, contributed to the Investigational New Drug application, and participated in weekly team meetings regarding the study.
Emmes, the statistical and data coordinating center for the study, developed the statistical analysis plan and performed all data analyses. Data reports, which were generated from the raw data by the statistical and data coordinating center, were provided and available to all the authors. The manuscript was written entirely by the authors, with the first two authors serving as overall lead authors. All the authors vouch for the completeness and accuracy of the data and for the adherence of the study to the protocol. No one who is not an author contributed to the writing of the manuscript.
The mRNA-1273 vaccine was administered as a 0.5-ml intramuscular injection into the deltoid on days 1 and 29 of the study; the same dose of the vaccine was administered on both days. Follow-up visits were scheduled 7 and 14 days after the administration of each dose of vaccine and on day 57. A standard toxicity scale was used to grade adverse events (Table S1 in the Supplementary Appendix, available at NEJM.org). Solicited local and systemic adverse events were collected for 7 days after each vaccination, as facilitated by the use of a memory aid. Data regarding unsolicited adverse events and the use of new medications were collected through day 57. Collection of specimens, as well as monitoring for medically attended adverse events, development of new chronic medical conditions, and serious adverse events, was scheduled to continue through 1 year after the last dose. These initial findings will be updated with final safety and immunogenicity data when the results are available.
After the initial safety data from the first phase of the study were available from participants between the ages of 18 and 55 years,2 the administration of mRNA-1273 was initiated sequentially in the subgroup of participants between the ages of 56 and 70 years at the 25-μg dose, which was followed by the initiation of the 100-μg dose. Since no halting rules were met after the participants in this subgroup had completed day 8, vaccine administration was initiated sequentially in the subgroup of participants who were 71 years of age or older at the 25-μg dose, which was followed by the initiation of the 100-μg dose.
Assessment of Antibody Responses
We performed enzyme-linked immunosorbent assays (ELISA) to quantify the binding IgG responses to S-2P containing an Asp (D) residue at position 614 (initial Wuhan-1 strain sequence8) and to the receptor-binding domain on days 1, 15, 29, 36, 43, and 57. (The receptor-binding domain is the portion of the SARS-CoV-2 virus that is located on its spike domain and that links with body receptors to infect cells.) A SARS-CoV-2 native spike-pseudotyped lentivirus reporter single-round-of-infection neutralization assay (pseudovirus neutralization assay) was used to assess vaccine-induced neutralizing activity against the 614D variant at the same time points. Vaccine-induced neutralization on day 43 was assessed with a second pseudovirus neutralization assay with the use of the 614-Gly (614G) polymorphic variant, since the 614G strain had become predominant in both the United States and worldwide.9 (Details are provided in the Methods section in the Supplementary Appendix.)
Three live-virus neutralization methods were used: first, the SARS-CoV-2 nanoluciferase high-throughput neutralization assay (nLuc HTNA), which uses a virus expressing the reporter gene nanoluciferase (nLuc)10; second, the focus reduction neutralization test mNeonGreen (FRNT-mNG), which uses recombinant SARS-CoV-2 expressing the fluorescent reporter gene mNeonGreen11; and third, a SARS-CoV-2 plaque-reduction neutralization testing (PRNT) assay, which uses wild-type virus. We used the nLuc HTNA to analyze specimens that were obtained on days 1, 29, and 43 from the participants who were 56 years of age or older and who received the 100-μg dose. We used the FRNT-mNG assay to analyze specimens obtained on days 1, 29, and 43 from all the participants in the two age and dose subgroups. For this preliminary report, because of the time-intensive nature of the PRNT assay and to maximize usable information obtained from its use, we performed PRNT assays for the presence of SARS-CoV-2 on samples obtained on days 1 and 43 from participants who received the 100-μg dose only. We used as comparators previously reported results for participants between the ages of 18 and 55 years who had been enrolled in the 100-μg subgroup, as well as results from controls who had donated convalescent serum.2 The severity of Covid-19 illness was known for 38 of these controls and was classified as mild in 63% of the participants, moderate in 22%, and severe (defined as hospitalization requiring intensive care, ventilation, or both) in 15%.
Assessment of T-Cell Responses
Intracellular cytokine-staining assays were performed to quantify antigen-specific T-cell responses against the spike protein on days 1, 29, and 43. (Details are provided in the Supplementary Appendix.)
Safety analyses included all the participants who had received at least one dose of mRNA-1273. Immunogenicity results excluded specimens that had been obtained after day 29 in a participant who had received only a single dose of vaccine. No other data points were missing. Seroconversion was defined as an increase from baseline in the antibody titer by a factor of 4 or more. Geometric means were calculated by log transforming the data points and calculating the mean and 95% confidence interval on the log-transformed data. The log-transformed mean and 95% confidence interval were then back-transformed to the original scale. We used the Student’s t-test to calculate confidence intervals. Interim analyses in the study subgroups were prespecified to inform critical decisions about vaccine development.