How often does vaccine-associated polio occur following oral polio vaccine?
Vaccine-associated paralytic polio (VAPP) is rare. From five to ten VAPP cases have been reported annually since live polio vaccine was licensed in 1963. From 1980 through 1991, 243 million doses of OPV were distributed and 98 total cases of VAPP were reported, for an overall risk of 1 case of VAPP per 2.5 million doses distributed. But the risk of VAPP varies by dose of vaccine. Among immunologically normal recipients of OPV during the same 12 year period there were 35 paralytic cases (an average of about 3 per year), for an overall risk of one VAPP case per 6.9 million OPV doses. However, among first dose recipients, the risk was 1 per 1.6 million doses. The risk for all other doses was one per 32.8 million doses. The reason for this difference by dose is probably because the vaccine virus is able to replicate longer in a completely nonimmune infant, thus increasing the risk of emergence of a mutant virus that may cause paralysis.
