The problem with current testing is that, while they provide some control groups usually those with rare reactions are not included in trials (because it may represent even less than 1 % of the population in a country). So even though there is deliberate screening for rare reactions, it is very likely that such events do not even get detected. The policy is that if serious reactions are found when the vaccine is in widespread use, the vaccine may be withdrawn. But by then it has made already damage and endangered human lives.  Further on, due to ethical concerns vaccine trials may deliberately exclude members of high risk groups and so prevent conclusive consequences for those groups. So while vaccines may be safe for the general population, they represent a high risk for parts of the population for which trials have not been done due to different reasons. [1]  [1] Why it is important to monitor vaccine safety, Center for disease control and Prevention,  , accessed 07/10/2011