Dumb question - what does ‘novel’ mean in relation to a vaccine? Is it just ‘new’? I don’t remember seeing the word used like that in other contexts.
As an adjective, it's applied to new approaches, new discoveries, previously uncharacterized behaviors, etc.
eg. https://scholar.google.com/scholar?q=novel+gene , https://scholar.google.com/scholar?q=novel+approach , etc.
When you think about it, publication is itself about presenting novel results. That's the goal, for better or worse.