Which vaccination is most likely to cause reversion?

• A. Polio
• B. Mumps
• C. Measles
• D. Hepatitis B

Answer: (A)

Reversion happens when a vaccine contains a live, weakened organism that can mutate back to a virulent form as it replicates in the body. The oral poliovirus vaccine uses a live, attenuated strain that replicates in the gut, so it can accumulate mutations and regain neurovirulence in rare cases. This is why vaccine-derived poliovirus or vaccine-associated paralytic poliomyelitis can occur, making polio vaccination the one with the notable reversion risk. Other vaccines mentioned do not carry this risk: the hepatitis B vaccine is a non-replicating recombinant protein, and the measles and mumps vaccines are live attenuated but have an extremely low, not practically relevant, chance of reversion.