Two proline amino acids in the spike protein lock it into the pre-fusion conformation, preventing cell membrane fusion.

“And the reason that is so important is because, when you're comparing the spike protein from the SARS-CoV-2 virus, as I mentioned, there's 26 of them on every viral particle, to the spike protein that is in the vaccines, including the mRNA vaccines and the vaccines in the United States, it's a different spike protein. It's a spike protein that cannot undergo that structural change. It does not elongate and, you know, dig into the cell membrane and fuse with it. It's a different spike protein because of those two proline amino acids that were inserted to lock it into the pre-fusion conformation.”