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Why did the United States switch from oral polio vaccine to inactivated polio vaccine in 2000?

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The United States transitioned from the oral poliovirus vaccine (OPV) to the inactivated poliovirus vaccine (IPV) in 2000 primarily to eliminate the risk of vaccine-associated complications.12

While OPV was highly effective in preventing polio, it contains live attenuated (weakened) virus that can, in rare instances, cause vaccine-associated paralytic polio (VAPP) in vaccine recipients or their close contacts. Additionally, the weakened virus used in OPV can mutate into vaccine-derived polioviruses (VDPVs), which are capable of causing paralysis.123

Because IPV is based on killed, or inactivated, virus, it provides protection against the disease without the risk of VAPP or the development of vaccine-derived strains. Consequently, high-income countries like the United States shifted to IPV to maintain polio-free status while ensuring the highest safety standards for their immunization programs.12