1.   The changes were made in large part because the oral vaccine in rare cases has caused polio in children who have weakened immune systems.

2.   The injected vaccine, which is just as effective as the oral one, cannot cause polio, because it uses an inactivated form of the virus.

3.   The injectable vaccine, with the inactivated virus that cannot cause polio, has been perfected.

4.   The Sabin vaccine carries a slight risk of causing polio either in the person who receives it, or in someone who comes in contact with a recent recipient.

5.   However, oral polio vaccine in rare instances does cause paralytic polio.

6.   Take the oral polio vaccine, a potent live-virus vaccine but one that carried a small risk of actually causing polio.

7.   Yet it carries a minute risk of causing polio, and produces the only known U.S. polio cases today.