Evaluation of the mRNA-1273 Vaccine against SARS-CoV-2 in Nonhuman Primates

Abstract

Background: Vaccines to prevent coronavirus disease 2019 (Covid-19) are urgently needed. The effect of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccines on viral replication in both upper and lower airways is important to evaluate in nonhuman primates.

Methods: Nonhuman primates received 10 or 100 μg of mRNA-1273, a vaccine encoding the prefusion-stabilized spike protein of SARS-CoV-2, or no vaccine. Antibody and T-cell responses were assessed before upper- and lower-airway challenge with SARS-CoV-2. Active viral replication and viral genomes in bronchoalveolar-lavage (BAL) fluid and nasal swab specimens were assessed by polymerase chain reaction, and histopathological analysis and viral quantification were performed on lung-tissue specimens.

Results: The mRNA-1273 vaccine candidate induced antibody levels exceeding those in human convalescent-phase serum, with live-virus reciprocal 50% inhibitory dilution (ID₅₀) geometric mean titers of 501 in the 10-μg dose group and 3481 in the 100-μg dose group. Vaccination induced type 1 helper T-cell (Th1)-biased CD4 T-cell responses and low or undetectable Th2 or CD8 T-cell responses. Viral replication was not detectable in BAL fluid by day 2 after challenge in seven of eight animals in both vaccinated groups. No viral replication was detectable in the nose of any of the eight animals in the 100-μg dose group by day 2 after challenge, and limited inflammation or detectable viral genome or antigen was noted in lungs of animals in either vaccine group.

Conclusions: Vaccination of nonhuman primates with mRNA-1273 induced robust SARS-CoV-2 neutralizing activity, rapid protection in the upper and lower airways, and no pathologic changes in the lung. (Funded by the National Institutes of Health and others.).

Figure 1. Antibody Responses after mRNA-1273 Vaccination in Rhesus Macaques.

Animals were administered phosphate-buffered saline (PBS) as a control or 10 μg or 100 μg of mRNA-1273. Serum specimens were assessed for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) S-specific IgG by enzyme-linked immunosorbent assay (ELISA) (Panel A) and SARS-CoV-2 pseudovirus neutralization (Panel B) at all time points after the first and second vaccinations. Data in Panel A are the area under the curve (AUC) and indicate the amount of IgG binding to S-2P over time, and data in Panel B are the reciprocal 50% inhibitory dilution (ID₅₀). Faint lines in Panels A and B represent individual animals, and bold lines represent the geometric mean titer for each group. S-specific IgG (Panel C), pseudovirus neutralization (Panel D), inhibition of angiotensin-converting enzyme 2 (ACE2) binding to the receptor-binding domain (RBD) (Panel E), and live-virus neutralization by NanoLuc reporter assay (Promega) (Panel F) were assessed at 4 weeks after the second vaccination, immediately before challenge. Results were compared with the antibody responses in a panel of human convalescent-phase serum specimens (Conv.) (42 specimens in Panels C, D, and E and 26 specimens in Panel F). In Panel E, the amount of signal emitted in wells containing no specimen was used as the maximal binding response against which each factor reduction was measured. In the box-and-whisker plots, the horizontal line indicates the median, the top and bottom of the box the interquartile range, and the whiskers the range. Symbols represent individual animals and overlap with one another for equal values where constrained. Dashed lines indicate the assay limit of detection.

Figure 2. T-Cell Responses after mRNA-1273 Vaccination in Rhesus Macaques.

Intracellular staining was performed on peripheral blood mononuclear cells at 8 weeks, immediately before challenge, to assess T-cell responses to the SARS-CoV-2 S1 peptide pool. Panel A shows type 1 helper T-cell (Th1) responses (interferon-γ, interleukin-2, or tumor necrosis factor α), Panel B Th2 responses (interleukin-4 or 13), Panel C CD40L up-regulation, and Panel D interleukin-21 from peripheral follicular helper T (Tfh) cells (central memory CXCR5+PD-1+ICOS+ CD4 T cells). Positivity with respect to intracellular cytokine responses was determined with the MIMOSA algorithm; numbers of animals positive and total numbers of animals are shown as fractions below each group. In the box-and-whisker plots, the horizontal line indicates the median, the top and bottom of the box the interquartile range, and the whiskers the range. Open symbols represent animals with a probable nonresponse, and solid symbols represent animals with a probable response. Dashed lines are used to highlight 0.0%.

Figure 3. Efficacy of mRNA-1273 against Upper and Lower Respiratory Viral Replication.

Bronchoalveolar-lavage (BAL) fluid (Panel A) and nasal swab (Panel B) specimens were obtained on days 1, 2, 4, and 7 after challenge, where applicable, and viral replication was assessed by analysis of SARS-CoV-2 subgenomic RNA. In the box-and-whisker plots, the horizontal line indicates the median, the top and bottom of the box the interquartile range, and the whiskers the range. Symbols represent individual animals and overlap with one another for equal values where constrained. Dashed lines indicate the assay limit of detection.

Figure 4. Lung Histopathological Analysis and Viral Detection 7 Days after Challenge in mRNA-1273–Vaccinated Rhesus Macaques.

Seven days after challenge, lungs were harvested from two animals per group for histopathological analysis and assessment of evidence of viral infection; representative images taken at different degrees of magnification are shown for localization of virus by chromogenic in situ hybridization (CISH) and SARS-CoV-2 immunohistochemical analysis (IHC) in serial tissue sections. The images in Panel B are shown at twice the magnification of the images in Panel A.