 This study developed a novel approach to identify mutations that can both increase the stability of a virus's pre-fusion form and improve its expression levels. The researchers used a combination of techniques including fluorescent fusion assays, mammalian cell display and deep mutagenesis screening to identify mutations that could be incorporated into a vaccine candidate. They found that certain mutations in the S2 domain of the SARS-CoV-2 spike protein were able to reduce the fusogenic activity of the protein while also increasing its stability in the pre-fusion state. These mutations could then be incorporated into a vaccine candidate to enhance its efficacy. This study provides a valuable tool for developing more effective vaccines against other viruses as well.