From what I can tell, it looks like the main danger is with a live vaccine, where the vaccine can give the disease to a large number of people (biggest actual disaster seems to have been the Cutter incident, which infected 40,000 people with polio).
I assume that the trial is also there to catch potential black swan issues.
IIRC the Covid-19 vaccines on trial are not live, so the case for doing the 14 month watch was not as strong as I expected. Certainly worth considering more carefully at least.
Whether or not a person gets a disease when you give them a live vaccine is information you learn very soon. It’s seems implossible to me that a live vaccine causes the virus 2 months afterwards let alone 12.
That sounds right to me.