A new protein-based vaccine candidate combined with a potent adjuvant provided effective protection against SARS-CoV-2 when tested in animals, suggesting that the combination could add one more promising COVID-19 vaccine to the list of candidates for human use. The protein antigen, based on the receptor binding domain (RBD) of SARS-CoV-2, was expressed in yeast instead of mammalian cells - which the authors say could enable a scalable, temperature-stable, low-cost production process well suited for deployment in the developing world. In a study by Maria Pino and colleagues, the adjuvant - a TLR7/TLR8 agonist named 3M-052, formulated with alum - substantially improved performance of the vaccine compared with vaccine adjuvanted with alum alone, inducing stronger antibody and T cell responses in vaccinated rhesus macaques. The vaccine and adjuvant combination also significantly reduced the quantity of virus in the respiratory tracts of macaques challenged by infection with SARS-CoV-2, and reduced lung inflammation as well. Pino et al. vaccinated 5 macaques with the RBD protein and the 3M-052/alum adjuvant and another 5 with the RBD protein and alum alone, each at 0, 4, and 9 weeks; they also included 5 unvaccinated macaques as controls. The vaccine and adjuvant combination induced more neutralizing antibodies with higher binding affinity for the virus RBD and also enhanced CD4+ and CD8+ T cell responses compared with the alum-only formulation. About one month after the third round of vaccinations, the researchers then infected the macaques with SARS-CoV-2, and noted the macaques vaccinated with the novel adjuvant formulation showed a reduced viral load in their nasal mucus and lung fluid, as well as fewer inflammatory cytokines in their plasma.