This inherent unpredictability problem is highlighted in the following scenario:
A coronavirus vaccine may not be dangerous initially. If the initial testing looks positive, mass vaccination efforts would presumably be administered to a large portion of the population.
In the first year or two, it may appear that there is no real safety issue, and over time, a greater percentage of the world population will be vaccinated due to this perceived “safety”. During this interim period, the virus is busy mutating. Eventually, the antibodies that vaccinated individuals have floating around in their bloodstream are now rendered non-neutralizing because they fail to bind to the virus with the same affinity due to the structural change resulting from the mutation.
Declining concentrations of the antibody over time would also contribute to this shift towards non-neutralization. When these previously vaccinate people are infected with this different strain of SARS-CoV-2, they could experience a much more severe reaction to the virus.
Bingo.. Your post nailed it.