The Sabin vaccine, formally known as the Oral Polio Vaccine (OPV), represents one of the most significant public health achievements of the 20th century. Developed by virologist Dr. Albert Sabin, it was introduced in the early 1960s and quickly became the primary weapon against poliomyelitis, a debilitating disease that once paralyzed hundreds of thousands of children annually. The use of this vaccine in mass immunization campaigns dramatically reduced the incidence of polio worldwide and brought the disease to the brink of eradication. Dr. Sabin refused commercial exploitation of his discovery, ensuring the vaccine could be produced at a low cost and remain accessible for global health initiatives.
How the Sabin Vaccine Works
The Sabin vaccine is classified as a live, attenuated vaccine. It contains a weakened form of the poliovirus that is still alive but modified to lose its ability to cause paralysis. The live virus is administered orally, typically as drops given to children, mimicking a natural infection within the gastrointestinal tract, the primary site of poliovirus entry. The attenuated virus replicates in the intestine, stimulating a robust local immune response that generates antibodies, including secretory Immunoglobulin A (IgA).
This intestinal, or mucosal, immunity distinguishes it from the injectable Salk vaccine, which uses an inactivated virus and primarily generates antibodies in the bloodstream. By replicating in the gut, the Sabin vaccine blocks subsequent poliovirus—either vaccine or wild strain—from multiplying and being shed in the feces. This action prevents the vaccinated person from transmitting the virus to others, which is the most powerful mechanism for stopping community spread. The attenuated strains were created to be neuroattenuated, meaning they cannot efficiently replicate in the central nervous system to cause paralytic disease.
Its Role in Polio Eradication
The unique properties of the Oral Polio Vaccine made it the preferred tool for the World Health Organization’s 1988 Global Polio Eradication Initiative. Oral administration is a logistical advantage because it does not require sterile needles or highly trained medical personnel. This allowed volunteers to easily administer it during mass vaccination campaigns in remote or resource-poor areas. The low cost, sometimes less than $0.15 per dose, further supported its use in global campaigns.
Another powerful epidemiological advantage is “secondary immunization,” or contact immunity. When the attenuated vaccine virus replicates, it is shed in the stool of the vaccinated individual. In areas with poor sanitation, this shed virus can circulate and inadvertently immunize unvaccinated contacts, boosting the community’s overall immunity. This ability to stop person-to-person transmission, coupled with ease of use, resulted in the OPV being administered more than 10 billion times. This led to a reduction in wild poliovirus cases by more than 99% since 1988.
Understanding Vaccine-Derived Poliovirus
The use of a live, attenuated virus introduces a rare but significant risk: the potential for the weakened virus to mutate back toward a virulent form. This genetic instability occurs during replication in the gut, leading to the emergence of neurovirulent strains known as Vaccine-Derived Polioviruses (VDPVs). There are two main concerns related to this risk: Vaccine-Associated Paralytic Polio (VAPP) and circulating Vaccine-Derived Poliovirus (cVDPV).
VAPP is an extremely rare event where the vaccine virus reverts and causes paralysis in the vaccinated individual or a close, non-immune contact. This occurs at a rate of approximately one case per 2.7 million doses administered, and VAPP does not lead to an outbreak. A greater threat is cVDPV, which emerges when the reverted virus circulates for an extended period in communities with low vaccination coverage. In these under-immunized populations, the virus regains the ability for sustained person-to-person transmission and can cause outbreaks clinically indistinguishable from the wild virus. The risk of cVDPV is why the OPV must eventually be phased out worldwide, as VDPV cases have exceeded those caused by the wild virus since 2014.
The Global Transition Strategy
The emergence of cVDPV, particularly from the type 2 component, necessitated a policy shift to secure a polio-free world. The strategy involves the phased withdrawal of the Oral Polio Vaccine, starting with the component responsible for the majority of vaccine-derived cases. The first step was the synchronized switch in April 2016 from the Trivalent OPV (tOPV), which contained types 1, 2, and 3, to the Bivalent OPV (bOPV), which contains only types 1 and 3.
This withdrawal of the type 2 poliovirus component was possible because wild poliovirus type 2 had been certified as eradicated. The ultimate goal is to transition entirely to the Inactivated Poliovirus Vaccine (IPV). IPV is injectable and contains killed virus that cannot revert to a virulent form, eliminating the risk of VDPV. While IPV protects the individual from paralysis, it is less effective at blocking intestinal infection, which is why OPV is still used where wild poliovirus transmission remains a threat. The final step of withdrawing all OPV will occur only after the eradication of all wild poliovirus types is confirmed.