No vaccine is 100% safe and effective. The safety and effectiveness is presumably high enough, though, or else they would not be used (or they have found no problems yet, possibly due to the funding and the commercial interests, etc; and/or if there are some bad effects but they are rare). Also presumably they are judged as being better than the alternatives. But, that does not mean that it is 100% good, because none of them are. But still, the time that has elapsed, and tests that have been made, if shows that it is OK, then it is less cause for concern than one which has not been tested yet and might have more severe side effects.
