Immune correlates of protection by mRNA-1273 vaccine against SARS-CoV-2 in nonhuman primates

Abstract

Immune correlates of protection can be used as surrogate endpoints for vaccine efficacy. Here, nonhuman primates (NHPs) received either no vaccine or doses ranging from 0.3 to 100 μg of the mRNA-1273 severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccine. mRNA-1273 vaccination elicited circulating and mucosal antibody responses in a dose-dependent manner. Viral replication was significantly reduced in bronchoalveolar lavages and nasal swabs after SARS-CoV-2 challenge in vaccinated animals and most strongly correlated with levels of anti–S antibody and neutralizing activity. Lower antibody levels were needed for reduction of viral replication in the lower airway than in the upper airway. Passive transfer of mRNA-1273–induced immunoglobulin G to naïve hamsters was sufficient to mediate protection. Thus, mRNA-1273 vaccine–induced humoral immune responses are a mechanistic correlate of protection against SARS-CoV-2 in NHPs.

mRNA-1273 vaccine–induced humoral immune responses are a mechanistic correlate of protection against SARS-CoV-2.

Levels of serum and mucosal spike-specific IgG in mRNA-1273–vaccinated NHPs were inversely correlated with the reduction of viral replication in the upper airway and lower airway after SARS-CoV-2 challenge.

Fig. 1.. Antibody responses after mRNA-1273 immunization.

(A to F) Rhesus macaques were immunized according to fig. S1 with PBS (gray) or mRNA-1273 (0.3 μg, green; 1 μg, purple; 3 μg, orange; 10 μg, blue; 30 μg, pink; 100 μg, red). Sera collected 4 weeks after boost, immediately before challenge, were assessed for SARS-CoV-2 S-specific (A) and RBD-specific (B) IgG by multi-array ELISA, inhibition of ACE2 binding to RBD (C), SARS-CoV-2 lentiviral-based pseudovirus neutralization (D), SARS-CoV-2 VSV-based pseudovirus neutralization (E), and SARS-CoV-2 EHC-83E focus reduction neutralization (F). (G to J) BAL [(G) and (I)] and nasal washes [(H) and (J)] collected 2 weeks after boost were assessed for SARS-CoV-2 S-specific IgG [(G) to (H)] and IgA [(I) to (J)] by multi-array ELISA. Squares represent NHPs in previous experiments (S1A, VRC-20-857.1; S1B, VRC-20-857.2) and circles represent individual NHPs in experiment S1C, VRC-20-857.4. Boxes and horizontal bars denote the interquartile range (IQR) and medians, respectively. Whisker end points are equal to the maximum and minimum values. Dotted lines indicate assay limits of detection, where applicable. NT, not tested. All measures were significantly correlated with dose (P < 0.0001), as determined by Spearman’s correlation test.

Fig. 2.. Correlations of humoral antibody analyses.

Rhesus macaques were immunized according to fig. S1C. Plots show correlations between SARS-CoV-2 S-specific IgG, RBD-specific IgG, ACE2-binding inhibition, lentiviral-based pseudovirus neutralization, VSV-based pseudovirus neutralization, and EHC-83E focus reduction neutralization at 4 weeks after boost. Circles represent individual NHPs, and colors indicate the mRNA-1273 dose as defined in fig. S1C. Dotted lines indicate assay limits of detection. Black and gray lines indicate linear regression and 95% confidence interval, respectively. r is Spearman’s correlation coefficient and P is the corresponding P value.

Fig. 3.. Efficacy of mRNA-1273 against upper and lower respiratory viral replication.

(A and B) Rhesus macaques were immunized and challenged as described in fig. S1C. BAL (A) and nasal swabs (NS) (B) were collected on days 2 (squares), 4 (triangles), and 7 (diamonds) after challenge, and viral replication was assessed by detection of SARS-CoV-2 N-specific sgRNA. In (A) and (B), Boxes and horizontal bars denote the IQR and medians, respectively. Whisker end points are equal to the maximum and minimum values. (C to E) Correlations shown between BAL and NS sgRNA at days 2 (C), 4 (D), and 7 (E) after challenge are Spearman’s correlation coefficients (r) and corresponding P values. Symbols represent an individual NHP and may overlap, i.e., n = 6 animals plotted at assay limit (dotted line) for both BAL and NS in (E).

Fig. 4.. Antibody correlates of protection.

Rhesus macaques were immunized and challenged as described in fig. S1C. Plots show correlations between SARS-CoV-2 N-specific sgRNA in BAL [(A) to (F)] and NS [(G) to (L)] at day 2 after challenge and before challenge (week 4 after boost) SARS-CoV-2 S-specific IgG [(A) and (G)], RBD-specific IgG [(B) and (H)], ACE2-binding inhibition [(C) and (I)], SARS-CoV-2 lentiviral-based pseudovirus neutralization [(D) and (J)], SARS-CoV-2 VSV-based pseudovirus neutralization [(E) and (K)], and SARS-CoV-2 EHC-83E focus reduction neutralization [(F) and (L)]. Gray shading for S-specific IgG represents the use of this assessment as primary predictor of protection outcome as stated in primary hypothesis. (M) Plot showing correlation between prechallenge (week 4 after boost) SARS-CoV-2 S-specific IgG with day 28 postchallenge SARS-CoV-2 N-specific IgG. Circles represent individual NHPs, and colors indicate the mRNA-1273 dose. Dotted lines indicate assay limits of detection. Black and gray lines indicate linear regression and 95% confidence interval, respectively. In (M), a red dotted horizontal line represents N-binding titers of 6, the maximum of all prechallenge values across all groups, and a red dotted vertical line represents a reciprocal S-specific IgG titer of 500, above which none of the animals had day 28 reciprocal N-binding titers >6.

Fig. 5.. Passive transfer of mRNA-1273 immune NHP IgG into Syrian hamsters.

(A) Sera were pooled from all NHPs that received 100 μg of mRNA-1273 in a primary vaccination series. (B) mRNA-1273 immune NHP IgG (2 μg, yellow; 10 μg, orange) or preimmune NHP IgG (10 μg, gray) was passively transferred to Syrian hamsters (n = 8/group) 24 hours before SARS-CoV-2 challenge. (C and D) Twenty-three hours after immunization, hamsters were bled to quantify circulating S-specific IgG (C) and SARS-CoV-2 pseudovirus-neutralizing antibodies (D). (E) After challenge, hamsters were monitored for weight loss. In (C) and (D), circles represent individual NHPs. Bars and error bars represent GMT and geometric SD, respectively. Asterisks at the axis represent animals that did not receive adequate IgG through passive transfer and were thus excluded from weight loss analyses. In (D), the dotted line indicates the neutralization assay limit of detection. In (E), circles and error bars represent means and SEM, respectively.