A major disadvantage of OPV is risk of what?

• A. It is completely safe with no risks.
• B. In rare cases, it can cause vaccine-associated paralytic poliomyelitis (VAPP).
• C. It does not pose any risk of paralysis.
• D. It is more expensive than IPV.

Answer: (B)

The main idea is that OPV uses a live, attenuated poliovirus. Because it is alive, the vaccine virus can, in rare cases, revert to a neurovirulent form and cause paralytic poliomyelitis—this is vaccine-associated paralytic poliomyelitis (VAPP). That risk is the major trade-off with OPV: it gives strong gut immunity and helps herd protection, but a tiny chance exists of paralysis from the vaccine virus itself. Inactivated polio vaccine (IPV) does not carry this risk because it uses killed virus. So the reason this is the best answer is that VAPP captures the fundamental disadvantage of OPV, distinguishing it from vaccines that cannot cause polio.