Could it increase cancer or other illnesses? Make kids sterile? They cant predict it. It may never happen and its all hunky dory which will be great but what if its not?
I don't think you understand the first thing about developing vaccines.
Scientists don't just throw a few chemicals in a mixer and see what comes out.
The ingredients are chosen specifically to do certain things, so along side the active ingredient(s) there will be things like preservatives.
Imagine you have a fabulous recipe for lemon cheese cake. You are asked to make an orange cheese cake. You have never made an orange cheesecake before.
You get your lemon cheese cake recipe and take out the lemon juice and swap it for orange juice.
Now until you taste it you do not know if it has worked but you do know a lot about it, how many calories are in it, that it won't cause food poisoning that it will last a few days in the fridge.
For some vaccines this is what happens, there is a successful recipe so swap the active ingredient and test.
Obviously it is not the same for all vaccines, I'm old enough to remember the race for an effective HIV vaccine, it doesn't exist, but effective treatment has been developed and we know more about the virus and why some people are naturally imune.