SARS‐CoV‐2 spike trimer vaccine expressed in Nicotiana benthamiana adjuvanted with Alum elicits protective immune responses in mice

The ongoing coronavirus disease 2019 (COVID‐19) pandemic has spurred rapid development of vaccines as part of the public health response. However, the general strategy used to construct recombinant trimeric severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) spike (S) proteins in mammalian cells is not completely adaptive to molecular farming. Therefore, we generated several constructs of recombinant S proteins for high expression in Nicotiana benthamiana. Intramuscular injection of N. benthamiana‐expressed S(ct) vaccine (NS(ct)Vac) into Balb/c mice elicited both humoral and cellular immune responses, and booster doses increased neutralizing antibody titres. In human angiotensin‐converting enzyme knock‐in mice, two doses of NS(ct)Vac induced anti‐S and neutralizing antibodies, which cross‐neutralized Alpha, Beta, Delta and Omicron variants. Survival rates after lethal challenge with SARS‐CoV‐2 were up to 80%, without significant body weight loss, and viral titres in lung tissue fell rapidly, with no infectious virus detectable at 7‐day post‐infection. Thus, plant‐derived NS(ct)Vac could be a candidate COVID‐19 vaccine.