Vaccine-associated paralytic poliomyelitis (VAPP) is best described as which of the following and its cause?

• A. An asymptomatic infection that never causes paralysis.
• B. A rare paralytic illness caused by reversion of Sabin vaccine strains to neurovirulent forms in the vaccinated person or contacts.
• C. A mild, self-limited illness following vaccination.
• D. A chronic, nonparalytic poliovirus infection.

Answer: (B)

VAPP arises because the oral polio vaccine uses live attenuated Sabin strains, which can rarely mutate back to a neurovirulent form as they replicate in the gut. If this reversion occurs in the vaccinated child or in a contact who becomes infected, the virus can invade the nervous system and cause paralytic poliomyelitis. This explains why it is described as a rare paralytic illness caused by reversion of Sabin vaccine strains to neurovirulent forms in the vaccinated person or contacts. The other scenarios—an asymptomatic infection, a mild self-limited illness after vaccination, or a chronic nonparalytic infection—do not produce paralysis and thus do not match VAPP.