Dual-antigen COVID-19 vaccination with oral boost protects NHP from viral challenge

ABSTRACT

Background: To address the need for an efficacious COVID-19 vaccine suitable for world-wide distribution, we have developed a dual-antigen vaccine incorporating genes for a modified SARS-CoV-2 spike protein (S-Fusion) and the viral nucleocapsid protein (N) with an Enhanced T-cell Stimulation Domain (N-ETSD) with the potential to increase MHC class I/II responses. The adenovirus serotype 5 platform used, hAd5 [E1-, E2b-, E3-] can be delivered in an oral formulation that overcomes cold-chain limitations. The hAd5 S-Fusion + N-ETSD vaccine was evaluated in rhesus macaques to determine both humoral and cell-mediated responses to vaccination, and protection from subsequent SARS-CoV-2 challenge. Methods: Non-human primates (NHP) received either a subcutaneous (SC) prime and two oral boosts at 2-week intervals, or one SC and one oral boost (each group n = 5). There was also a placebo group (n = 2). Humoral responses to spike (S) were determined by ELISA and T-cell responses to S and nucleocapsid (N) by ELISpot. Neutralization capability of sera was assessed by a surrogate assay and by a microneutralization assay. After SARS-CoV-2 challenge of 10e6 TCID50, genomic RNA (gRNA) and subgenomic RNA (sgRNA) were determined in nasal swab and bronchoalveolar lavage (BAL) samples by RT qPCR. Results: In response to hAd5 S-Fusion + N-ETSD vaccination, NHP generated SARS-CoV-2-neutralizing anti-spike (S) antibodies and demonstrated T-cell activation by both S and nucleocapsid (N). Both the subcutaneous (SC) prime followed by two oral boosts or an SC and oral boost protected the upper and lower respiratory tracts of non-human primates from high titer SARS-CoV-2 challenge. Notably, inhibition of viral replication began within 24 hours of challenge in both lung and nasal passages, becoming undetectable within 7 days post-challenge. Rapidly enhanced neutralization capability of sera in the two weeks after challenge suggests the presence of memory B cells that were activated by infection. Conclusion: The hAd5 S-Fusion + N-ETSD vaccine, when give as a subcutaneous prime with oral boosts, protects rhesus macaques from subsequent viral challenge. The decrease in subgenomic RNA seen at the first time point for sample collection post-challenge (24 hours) provides evidence that protection was almost immediate. The thermally-stable oral form of the vaccine has the potential to facilitate global distribution of vaccines, especially in developing nations.