History of Polio and IPV Vaccines
Poliomyelitis, commonly known as polio, is a disabling and life-threatening disease caused by the poliovirus. The virus spreads from person to person and can infect a person's spinal cord, causing paralysis. For decades, polio was one of the most feared diseases in industrialized nations, affecting primarily young children. Major epidemics of polio occurred regularly during the first half of the 20th century. In 1952, nearly 58,000 cases were reported in the United States alone. Jonas Salk developed the first effective vaccine against polio in 1955, and it was a live attenuated oral polio vaccine (OPV). This was followed in 1961 by Albert Sabin's live attenuated OPV. While effective, these early vaccines carried some risk of vaccine-associated paralytic poliomyelitis. It was not until the development of inactivated poliovirus vaccine (IPV) in the 1960s that a injectable vaccine free of infectious virus became available.

Changes to Global Polio Eradication Strategies

The global effort to eradicate polio has focused primarily on the use of OPV. However, the inherent limitations of OPV use have led to changes in strategy. Rarely, the attenuated live virus in OPV can mutate into a form capable of causing new outbreaks of polio. This vaccine-derived poliovirus (VDPV) has caused several outbreaks globally. In addition, in a small percentage of cases, the oral vaccine itself can cause vaccine-associated paralytic poliomyelitis. To address these risks, the World Health Organization switched its recommendation in all polio-endemic countries from trivalent OPV to bivalent OPV without the type 2 component in 2016. This removed the opportunity for circulating vaccine-derived poliovirus type 2 emergences.

Starting in late 2015, more than 150 countries have switched from trivalent OPV to IPV in their routine immunization programs. Countries used at least 1 dose of IPV concurrently with any dose of OPV to improve population immunity. IPV provides active immunity against polio without using any live attenuated virus. This reduces VDPV risk and stops vaccine-associated paralytic poliomyelitis. In April 2016, a global switch was completed to remove Sabin 2 from trivalent OPV worldwide. The polio endgame strategy now relies primarily on IPV use with a focus on improving responsive immunization campaigns to find and stop any new outbreaks.

Advances in IPV Production and Delivery

Several innovations aim to improve the impact and affordability of IPV as the key tool for finishing polio eradication. Traditionally, IPV Vaccines is produced through a longer and more complex manufacturing process than OPV. Leading vaccine makers are pursuing new cell culture and advanced purification techniques that can increase production capacity for IPV while lowering costs. Novel adjuvants and vaccine delivery methods may also enhance IPV's protective immune responses. Some new options now in testing include nanotechnology-enabled IPV patches and thermostable liquid IPV that does not require cold chain. Successful introduction of these improved IPV products could help overcome challenges like maintaining high vaccination rates in hard-to-reach populations.

Scientists have also worked to develop more immunogenic fractional-dose IPV (fIPV) schedules. Full-dose IPV requires four injections to complete the series. In late 2018, WHO recommended that countries using IPV introduce at least one dose of fIPV. Clinical studies show fIPV administered intradermally or as a lyophilized formulation presents non-inferior immunity compared to standard IPV while stretching supply. This can support vaccination campaigns during the polio endgame by simplifying logistics and lowering costs. Researchers continue optimizing fIPV dosing schedules to maximize population immunity.

Transition to IPV-only Strategies

As a result of these innovations, more polio-free regions now plan transitions to exclusive IPV use in routine immunization as a long-term strategy. Without OPV circulation, the chance of new VDPV outbreaks essentially becomes zero. This reduces global polio risks substantially. Countries like the U.S., Canada, Australia and much of Western Europe have relied solely on IPV schedules for years. Their continued success demonstrates that high population immunity levels can be maintained effectively with IPV alone. India switched from trivalent OPV to IPV-only in 2016 after a sustained lack of wild polio cases. Other regions of the world are considering similar IPV transitions once they achieve WHO certification standards as polio-free.

The final stage of global polio eradication relies on stopping all OPV2 use and transitioning all polio-endemic and high-risk countries to exclusively IPV. This worldwide “OPV2 withdrawal” occurred on a coordinated schedule in April-May 2016. The goal was protects populations from the rare but serious risks of circulating VDPVs while immunization infrastructure focuses IPV delivery to high-priority areas and catch-up campaigns to consolidate population immunity gains.

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