There are no absolutely safe drugs, all medical treatments have possibilities of developing into complications. Just like penicillin can cause problem in some but not general others. All drugs in current western medicine underwent clincial trials to make sure they are safe to most population out there, but as we humans are so diverse, there is bound to have people with adverse effect from what seemed good. Its just simple maths, say 1 in a million will have complications, with 100 millions people taking a vaccine, you will likely see 100 people suffer, but please don't forget the 99,999,990 being protected (honestly no one reports that on the news)