Disease evolve so there is always going to be some unpredictable factor so a vaccines effectiveness long term cannot be predicted for certain. But when it comes to long term side effects I think scientists have much better ideas of how the ingredients in the vaccine are going to interact with our biology.
It wasn't long ago that airplanes had to be drawn by hand and wind tunnels be used for extensive testing. Now with computers these things can all be simulated with much higher confidence and speed.
I never said anything "can be predicted for certain." No doctor in his right mind guarantees a given result from any medication. There's alway a certain degree of risk that drugs will not work or will cause worse problems for a given patient. What they can do is offer an OPINION on the risks. It's up to the patient to accept them, not the doctors.
I went for LASIK surgery. The first visit was to identify whether or not I was a viable candidate for the surgery. Next visit was to map my eyes with a computer and create a custom prescription for each lens. Next visit was to actually cut my lenses with the laser. The day following surgery, I was tested with 20/20 in the right, and 20/10 in the left.
Before I agreed with the procedure, I had to read some literature and watch a video outlining all the bad things that some LASIK patients have experienced. The effects included worse vision than before the surgery, constant halo effects from lights at night, and even blindness.
In the end, I had to make the decision based on the information provided.
As of now, there's not enough information for the individual to make a truly informed decision, because there hasn't been enough time to really know all the effects of the vaccines -- no matter how many drug-maker-sponsored studies the government cites.