From the last paragraph of the paper:
“The current versions of the Moderna, Pfizer-BioNTech, and Johnson & Johnson SARS-CoV-2 vaccines encode the full-length spike S protein with two mutations (spike S-2P) that stabilize the prefusion conformation and inactivate its fusogenicity (39, 64, 67, 68). We used this same mutant form of spike S-2P as a negative control, demonstrating the complete lack of fusogenicity when two consecutive prolines were added at positions 986 and 987. However, our findings demonstrate that it will be critical to consider the fusogenic potential when designing any future vaccines in which viral fusogens are to be expressed in mammalian cells.”