What are disadvantages of oral polio vaccine?

Answer 1

The oral polio vaccine is a live virus that is mutated in order for it to be strong enough to induce an immune response but weak enough to not cause disease. This replication in the host of the live virus gave a better immune response and also allowed the vaccine strain to spread and inoculate people who did not directly (intentionally) received the disease. The problem is that rarely it could "back mutate" and actually produce polio disease.

It is believed that when the oral polio vaccine was widely used in the US there were about 10 cases per year of vaccine induced polio (compared to thousands of cases per year of natural polio before the vaccine was developed). Some studies cast doubt on this number by carefully examining proposed cases and finding other causes in most of them.

When the polio incidence became nearly zero it was decided that the advantaged of increased immunity and increased spread of the oral vaccine were not worth the vaccine induced cases and so the US, and most developed countries, switched to the injectable vaccine.