I think it’s the virus itself, and is fully contagious.
Remember how vaccines work, basically they inject killed viruses into the body. Your bodies immune system sees the viral envelope, membrane, and you develop antibodies that can now see the virus and destroy it on contact.
It’s not hard to believe that sometimes, not often, they don’t kill ALL the viruses, and a live one slips through.
I read the AP version {LA CBS Station} of this story. The doctors claim it’s not Polio and that onset to full severity happens within two days. That sounds just like how Transverse Myelitis does. The onset can be as mild as a crick in the neck. The fact they aren’t getting much limb function back even more so sounds like it. TM and Polio are very simular in nature. If it were the Polio Virus they’d know real fast. They may never pin point the cause.