Test project and participants
We initially conducted an open-label, dose-escalating Phase 1 clinical study of mRNA-1273 involving participants aged 18 to 55 years.2 in which we evaluated doses of 25 μg, 100 μg and 250 μg. We then expanded the study to include 40 participants aged 56 and over and who were stratified into two subgroups: those aged 56 to 70 and those aged 71 and over. Due to the clinically significant systemic reactogenicity observed in participants aged 18 to 55 years at the 250 μg dose, we administered either 25 μg or 1
The study was conducted at the 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 prior to undergoing any study procedures. We did not examine evidence of past or current SARS-CoV-2 infection by testing blood or nasal samples prior to enrollment. Full eligibility criteria, along with details of study design, conduct, supervision, and statistical analyzes, are described in the protocol, available with the full text of this article on NEJM.org.
The mRNA-1273 vaccine was developed 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-tip 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.
Supervision of the study
NIAID sponsored the trial and made all decisions regarding the design and implementation of the study. The Investigational New Drug Vaccine Application and Protocol Amendment Expanding Age Subgroups was reviewed by the Food and Drug Administration and the Institutional Review Board of Advarra, a regulatory compliance consultancy, which served as a single committee institutional review for all study sites. An independent data and safety monitoring committee reviewed the interim safety reports.
Moderna provided mRNA-1273 for use in this study, but did not provide any financial support. Moderna employees collaborated in the development of the protocol, contributed to the application of the new investigational drug and participated in weekly team meetings regarding the study.
Emmes, the statistical and data coordination center for the study, developed the statistical analysis plan and performed all data analyzes. Data reports, which were generated from raw data by the statistical and data coordination center, were provided and available to all authors. The manuscript was written entirely by the authors, with the first two authors acting as lead authors. All authors guarantee the completeness and accuracy of the data and 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 given as a 0.5 ml intramuscular injection into the deltoid on days 1 and 29 of the study; the same vaccine dose was given on both days. Follow-up visits were scheduled 7 and 14 days after the administration of each vaccine dose and on day 57. A standard toxicity scale was used to classify adverse events (Table S1 in the Supplementary Appendix, available on NEJM .org). Required local and systemic adverse events were collected for 7 days after each vaccination, as facilitated by the use of a memory aid. Data on unsolicited adverse events and new drug use were collected up to day 57. Specimen collection, as well as physician-assisted adverse event monitoring, development of new chronic medical conditions and adverse events severe, was scheduled to continue for 1 year after the last dose. These initial results will be updated with final safety and immunogenicity data when the results become available.
After initial safety data from the first phase of the study were available from participants aged 18 to 55 years,2 administration of mRNA-1273 was sequentially initiated in the subgroup of participants aged 56 to 70 years at the 25 μg dose, followed by the initiation of the 100 μg dose. Because discontinuation rules were not adhered to after participants in this subgroup had completed day 8, vaccine administration was initiated sequentially in the subgroup of participants 71 years of age and older at the 25 μg dose, which is was followed by the start of the 100 μg dose.
Evaluation of antibody responses
We performed enzyme-linked immunosorbent assays (ELISA) to quantify S-2P binding IgG responses 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 on its peak domain and which connects with body receptors to infect cells.) A single-round neutralization test of lentivirus reporter infection with SARS-CoV-2 peak pseudotype (pseudovirus neutralization test) was used to evaluate the neutralizing activity induced by the vaccine against the 614D variant at the same time points. Vaccine-induced neutralization at day 43 was evaluated with a second pseudovirus neutralization test using the polymorphic variant 614-Gly (614G), as the 614G strain had become predominant in both the United States and the world.9 (Details are provided in the Methods section of 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 that expresses the nanoluciferase (nLuc) reporter gene10; second, the mNeonGreen focus reduction neutralization assay (FRNT-mNG), which uses recombinant SARS-CoV-2 expressing the mNeonGreen fluorescent reporter gene11; and third, a SARS-CoV-2 Plaque Reduction Neutralization (PRNT) assay, which uses wild-type viruses. We used HTNA nLuc to analyze samples obtained on Days 1, 29, and 43 from participants aged 56 years and older and who received the 100 μg dose. We used the FRNT-mNG assay to analyze samples obtained on days 1, 29 and 43 from all participants in the two age and dose subgroups. For this preliminary report, due to the laborious nature of the PRNT test and to maximize the usable information obtained from its use, we performed PRNT tests for the presence of SARS-CoV-2 on samples obtained on days 1 and 43 from participants who only received the 100 μg dose. We used as comparators the previously reported results for participants aged 18 to 55 who were enrolled in the 100 μg subgroup, as well as the results from controls who donated convalescent serum.2 The severity of Covid-19 disease was known for 38 of these controls and was classified as mild in 63% of participants, moderate in 22% and severe (defined as hospitalization requiring intensive care, ventilation, or both) in 15%.
Evaluation of T cell responses
Intracellular cytokine staining tests 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 analyzes included all participants who had received at least one dose of mRNA-1273. The immunogenicity results excluded samples that were obtained after day 29 in a participant who received only a single dose of the vaccine. There was no lack of other data points. Seroconversion was defined as an increase from baseline in antibody titer by a factor of 4 or more. Geometric means were calculated using logs by transforming the data points and calculating the mean and the 95% confidence interval on the transformed log data. The log-transformed mean and the 95% confidence interval were then converted back to the original scale. We used Student’s t-test to calculate confidence intervals. The interim analyzes in the study subgroups were prespecified to inform critical decisions about vaccine development.