An mRNA Vaccine against SARS-CoV-2 — Preliminary Report

Trial Design and Participants

We conducted a phase 1, dose-escalation, open-label clinical trial designed to determine the safety, reactogenicity, and immunogenicity of mRNA-1273. Eligible participants were healthy adults 18 to 55 years of age who received two injections of trial vaccine 28 days apart at a dose of 25 μg, 100 μg, or 250 μg. On the basis of the results obtained in patients at these dose levels, additional groups were added to the protocol; those results will be reported in a subsequent publication. Participants were not screened for SARS-CoV-2 infection by serology or polymerase chain reaction before enrollment. The trial was conducted at the Kaiser Permanente Washington Health Research Institute in Seattle and at the Emory University School of Medicine in Atlanta. The protocol, available with the full text of this article at NEJM.org, permitted interim analyses to inform decisions regarding vaccine strategy and public health; this interim analysis reports findings through day 57. Full details of the trial design, conduct, oversight, and analyses can be found in the protocol and statistical analysis plan (available at NEJM.org).

The trial was reviewed and approved by the Advarra institutional review board, which functioned as a single board and was overseen by an independent safety monitoring committee. All participants provided written informed consent before enrollment. The trial was conducted under an Investigational New Drug application submitted to the Food and Drug Administration. The vaccine was codeveloped by researchers at the National Institute of Allergy and Infectious Diseases (NIAID, the trial sponsor) and at Moderna (Cambridge, MA). Moderna was involved in discussions of the trial design, provided the vaccine candidate, and, as part of the writing group, contributed to drafting the manuscript. The Emmes Company, as a subcontractor to the NIAID, served as the statistical and data coordinating center, developed the statistical analysis plan, and performed the analyses. The manuscript was written entirely by the authors, with the first author as the overall lead author, the fourth author as the lead NIAID author, and the last two authors as senior authors (details are provided in the Supplementary Appendix, available at NEJM.org). The authors had full access to the data reports, which were prepared from the raw data by the statistical and data coordinating center, and vouch for the completeness and accuracy of the data and for the fidelity of the trial to the protocol.

Vaccine

The mRNA-1273 vaccine candidate, manufactured by Moderna, encodes the S-2P antigen, consisting of the SARS-CoV-2 glycoprotein with a transmembrane anchor and an intact S1–S2 cleavage site. S-2P is stabilized in its prefusion conformation by two consecutive proline substitutions at amino acid positions 986 and 987, at the top of the central helix in the S2 subunit.⁸ The lipid nanoparticle capsule composed of four lipids was formulated in a fixed ratio of mRNA and lipid. The mRNA-1273 vaccine was provided as a sterile liquid for injection at a concentration of 0.5 mg per milliliter. Normal saline was used as a diluent to prepare the doses administered.

Trial Procedures

The vaccine was administered as a 0.5-ml injection in the deltoid muscle on days 1 and 29; follow-up visits were scheduled for 7 and 14 days after each vaccination and on days 57, 119, 209, and 394. The dose-escalation plan specified enrollment of four sentinel participants in the 25-μg group, followed by four sentinel participants in the 100-μg group, followed by full enrollment of those two dose groups. If no halting rules were met after all participants in those two dose groups completed day 8, four sentinel participants in the 250-μg group were enrolled, followed by the remainder of that dose group.

Participants recorded local and systemic reactions, using a memory aid, for 7 days after each vaccination. Participants were not instructed to routinely use acetaminophen or other analgesics or antipyretics before or after the vaccinations but were asked to record any new medications taken. Adverse events were graded according to a standard toxicity grading scale (Table S1 in the Supplementary Appendix).⁹

Assessment of SARS-CoV-2 Binding Antibody and Neutralizing Responses

Binding antibody responses against S-2P and the isolated receptor-binding domain, located in the S1 subunit, were assessed by enzyme-linked immunosorbent assay (ELISA). Vaccine-induced neutralizing activity was assessed by a pseudotyped lentivirus reporter single-round-of-infection neutralization assay (PsVNA) and by live wild-type SARS-CoV-2 plaque-reduction neutralization testing (PRNT) assay. ELISA and PsVNA were performed on specimens collected from all participants on days 1, 15, 29, 36, 43, and 57. Because of the time-intensive nature of the PRNT assay, for this report of the interim analysis, results were available only for the day 1 and day 43 time points in the 25-μg and 100-μg dose groups.

For comparison of the participants’ immune responses with those induced by SARS-CoV-2 infection, 41 convalescent serum specimens were also tested. The assays were performed at the NIAID Vaccine Research Center (ELISA and PsVNA) and the Vanderbilt University Medical Center (PRNT).

Assessment of SARS-CoV-2 T-Cell Responses

T-cell responses against the spike protein were assessed by an intracellular cytokine–staining assay, performed on specimens collected at days 1, 29, and 43. For this report of the interim analysis, results were available only for the 25-μg and 100-μg dose groups. These assays were performed at the NIAID Vaccine Research Center. (See the Supplementary Appendix for details of all assay methods and for characteristics of the convalescent serum specimens.)

Statistical Analysis

Results of immunogenicity testing of the 45 enrolled participants excluded findings for day 36, day 43, and day 57 for 3 participants who did not receive the second vaccination and for time points at which specimens were not collected (in the 100-μg group: 1 participant at day 43 and day 57; in the 250-μg group: 1 participant at day 29 and 1 at day 57). Confidence intervals of the geometric means were calculated with the Student’s t distribution on log-transformed data. Seroconversion as measured by ELISA was defined as an increase by a factor of 4 or more in antibody titer over baseline.

Trial Population

The 45 enrolled participants received their first vaccination between March 16 and April 14, 2020 (Fig. S1). Three participants did not receive the second vaccination, including one in the 25-μg group who had urticaria on both legs, with onset 5 days after the first vaccination, and two (one in the 25-μg group and one in the 250-μg group) who missed the second vaccination window owing to isolation for suspected Covid-19 while the test results, ultimately negative, were pending. All continued to attend scheduled trial visits. The demographic characteristics of participants at enrollment are provided in Table 1.

Vaccine Safety

No serious adverse events were noted, and no prespecified trial halting rules were met. As noted above, one participant in the 25-μg group was withdrawn because of an unsolicited adverse event, transient urticaria, judged to be related to the first vaccination.

After the first vaccination, solicited systemic adverse events were reported by 5 participants (33%) in the 25-μg group, 10 (67%) in the 100-μg group, and 8 (53%) in the 250-μg group; all were mild or moderate in severity (Figure 1 and Table S2). Solicited systemic adverse events were more common after the second vaccination and occurred in 7 of 13 participants (54%) in the 25-μg group, all 15 in the 100-μg group, and all 14 in the 250-μg group, with 3 of those participants (21%) reporting one or more severe events.

None of the participants had fever after the first vaccination. After the second vaccination, no participants in the 25-μg group, 6 (40%) in the 100-μg group, and 8 (57%) in the 250-μg group reported fever; one of the events (maximum temperature, 39.6°C) in the 250-μg group was graded severe. (Additional details regarding adverse events for that participant are provided in the Supplementary Appendix.)

Local adverse events, when present, were nearly all mild or moderate, and pain at the injection site was common. Across both vaccinations, solicited systemic and local adverse events that occurred in more than half the participants included fatigue, chills, headache, myalgia, and pain at the injection site. Evaluation of safety clinical laboratory values of grade 2 or higher and unsolicited adverse events revealed no patterns of concern (Supplementary Appendix and Table S3).

SARS-CoV-2 Binding Antibody Responses

Binding antibody IgG geometric mean titers (GMTs) to S-2P increased rapidly after the first vaccination, with seroconversion in all participants by day 15 (Table 2 and Figure 2A). Dose-dependent responses to the first and second vaccinations were evident. Receptor-binding domain–specific antibody responses were similar in pattern and magnitude (Figure 2B). For both assays, the median magnitude of antibody responses after the first vaccination in the 100-μg and 250-μg dose groups was similar to the median magnitude in convalescent serum specimens, and in all dose groups the median magnitude after the second vaccination was in the upper quartile of values in the convalescent serum specimens. The S-2P ELISA GMTs at day 57 (299,751 [95% confidence interval {CI}, 206,071 to 436,020] in the 25-μg group, 782,719 [95% CI, 619,310 to 989,244] in the 100-μg group, and 1,192,154 [95% CI, 924,878 to 1,536,669] in the 250-μg group) exceeded that in the convalescent serum specimens (142,140 [95% CI, 81,543 to 247,768]).

SARS-CoV-2 Neutralization Responses

No participant had detectable PsVNA responses before vaccination. After the first vaccination, PsVNA responses were detected in less than half the participants, and a dose effect was seen (50% inhibitory dilution [ID₅₀]: Figure 2C, Fig. S8, and Table 2; 80% inhibitory dilution [ID₈₀]: Fig. S2 and Table S6). However, after the second vaccination, PsVNA responses were identified in serum samples from all participants. The lowest responses were in the 25-μg dose group, with a geometric mean ID₅₀ of 112.3 (95% CI, 71.2 to 177.1) at day 43; the higher responses in the 100-μg and 250-μg groups were similar in magnitude (geometric mean ID₅₀, 343.8 [95% CI, 261.2 to 452.7] and 332.2 [95% CI, 266.3 to 414.5], respectively, at day 43). These responses were similar to values in the upper half of the distribution of values for convalescent serum specimens.

Before vaccination, no participant had detectable 80% live-virus neutralization at the highest serum concentration tested (1:8 dilution) in the PRNT assay. At day 43, wild-type virus–neutralizing activity capable of reducing SARS-CoV-2 infectivity by 80% or more (PRNT₈₀) was detected in all participants, with geometric mean PRNT₈₀ responses of 339.7 (95% CI, 184.0 to 627.1) in the 25-μg group and 654.3 (95% CI, 460.1 to 930.5) in the 100-μg group (Figure 2D). Neutralizing PRNT₈₀ average responses were generally at or above the values of the three convalescent serum specimens tested in this assay. Good agreement was noted within and between the values from binding assays for S-2P and receptor-binding domain and neutralizing activity measured by PsVNA and PRNT (Figs. S3 through S7), which provides orthogonal support for each assay in characterizing the humoral response induced by mRNA-1273.

SARS-CoV-2 T-Cell Responses

The 25-μg and 100-μg doses elicited CD4 T-cell responses (Figs. S9 and S10) that on stimulation by S-specific peptide pools were strongly biased toward expression of Th1 cytokines (tumor necrosis factor α > interleukin 2 > interferon γ), with minimal type 2 helper T-cell (Th2) cytokine expression (interleukin 4 and interleukin 13). CD8 T-cell responses to S-2P were detected at low levels after the second vaccination in the 100-μg dose group (Fig. S11).