Variant SARS-CoV-2 mRNA vaccines confer broad neutralization as primary or booster series in mice

Abstract

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of a global pandemic. Safe and effective COVID-19 vaccines are now available, including mRNA-1273, which has shown 94% efficacy in prevention of symptomatic COVID-19 disease. However, the emergence of SARS-CoV-2 variants has led to concerns of viral escape from vaccine-induced immunity. Several variants have shown decreased susceptibility to neutralization by vaccine-induced immunity, most notably B.1.351 (Beta), although the overall impact on vaccine efficacy remains to be determined. Here, we present the initial evaluation in mice of 2 updated mRNA vaccines designed to target SARS-CoV-2 variants: (1) monovalent mRNA-1273.351 encodes for the spike protein found in B.1.351 and (2) mRNA-1273.211 comprising a 1:1 mix of mRNA-1273 and mRNA-1273.351. Both vaccines were evaluated as a 2-dose primary series in mice; mRNA-1273.351 was also evaluated as a booster dose in animals previously vaccinated with mRNA-1273. The results demonstrated that a primary vaccination series of mRNA-1273.351 was effective at increasing neutralizing antibody titers against B.1.351, while mRNA-1273.211 was effective at providing broad cross-variant neutralization. A third (booster) dose of mRNA-1273.351 significantly increased both wild-type and B.1.351-specific neutralization titers. Both mRNA-1273.351 and mRNA-1273.211 are being evaluated in pre-clinical challenge and clinical studies.

Keywords: Booster dose; COVID-19; Neutralization; Primary series; SARS-CoV-2 variants of concern; mRNA-1273.

Fig. 1

Model of S protein. mRNA-1273.351 encodes the B.1.351 lineage S variant. Surface representation of the trimeric S protein in the vertical view with the locations of surface-exposed mutated residues highlighted in red spheres and labelled on the gray monomer. The inset shows superimposition of ACE2 receptor domain and the RBD. S protein structure, 6VSB . ACE2-RBD structure, 6M0J . ACE2 = angiotensin-converting enzyme 2; NTD = N-terminal domain; RBD = receptor binding domain.

Fig. 2

S protein-binding antibody and neutralization of variant SARS-CoV-2 pseudoviruses by serum from vaccinated BALB/c mice. a, BALB/c mice were immunized on a two-dose schedule with 1 or 10 µg mRNA-1273, mRNA-1273.351, mRNA-1273.211 (1:1 mix of mRNA-1273 and mRNA-1273.351), or PBS. b, Results from individual mouse sera (n = 10) following dose 1 (day 15) and after dose 2 (day 36) are represented as dots on each figure; the horizontal line indicates the GMT. c, BALB/c mice were immunized with 1 µg mRNA-1273, mRNA-1273.351, or mRNA-1273.211. Each bar indicates the GMT value after dose 2 (day 36), which is listed as text above each plot. d, GMT values from individual mouse sera (n = 8 per antigen [randomly selected]) after dose 2 (day 36) of 1 µg mRNA-1273, mRNA-1273.351, or mRNA-1273.211 are represented as dots on each figure, with lines connecting matched pairs for the D614G and B.1.351 neutralization titers. Fold difference in neutralization against each virus was shown in text above each plot. Wilcoxon matched-pairs signed-rank test. Two-tailed p values: *<0.1; **<0.01. e, BALB/c mice were immunized with 1 µg mRNA-1273, mRNA-1273.351, or the multivalent mRNA-1273.211 vaccine (n = 8). Each bar indicates the GMT value after dose 2 (day 36), which is listed as text above each plot. The horizontal dotted line indicates the lower limit of quantitation for NAb titer at 40 ID₅₀. ELISA = enzyme-linked immunosorbent assay; GMT = geometric mean titer; ID₅₀ = inhibitory dilution factor; IgG = immunoglobulin G; Nab = neutralizing antibody; ns, not significant; PBS = phosphate-buffered saline; PsVN = pseudovirus neutralization titer.

Fig. 3

S protein-binding antibody and neutralization of D614G and B.1.351 SARS-CoV-2 pseudoviruses by serum from 1 µg mRNA-1273.351 boosted BALB/c mice. a, BALB/c mice were immunized with 1 or 0.1 µg mRNA-1273 (dose 1 on day 1; dose 2 on day 22) and were boosted with 1 or 0.1 µg mRNA-1273.351 on day 213. b, Results from individual mouse sera (n = 5 per group) are represented as dots on each figure, and the line is the mean of each group. The horizontal dotted line indicates the lower limit of quantitation for log₁₀ IgG titer at 1.398. c, BALB/c mice previously immunized with mRNA-1273 were given a third dose with 1 µg mRNA-1273.351, with PsVN assessed against wild-type D614G and B.1.351 prior to dose 3 (day 212) and 3 weeks after dose 3 (day 233). Postdose 2 peak neutralization titer (day 36) was referenced against D614G. d, Fold rise in neutralization against both viruses from the boosting dose of mRNA-1273.351. e, Fold difference in neutralization prior to and after dose 3. Postdose 2 peak neutralization titer reference (D614G assay). The box indicates the GMT, which is listed as text above each plot. The horizontal dotted line indicates the lower limit of quantitation for NAb titer at 40 ID₅₀. Results from individual mouse sera are represented as dots on each figure, with lines connecting the D614G and B.1.351 neutralization titers. ELISA = enzyme-linked immunosorbent assay; GMT = geometric mean titer; ID₅₀ = inhibitory dilution factor; IgG = immunoglobulin G; NAb = neutralizing antibody; ns, not significant; PBS = phosphate-buffered saline; PsVN = pseudovirus neutralization titer.