Polio, a serious infectious disease, was once a widespread cause of paralysis and even death, particularly affecting children. Global vaccination efforts have substantially decreased polio incidence by over 99% since 1988. Despite this progress, questions arise about vaccine protection: can an individual still contract polio after vaccination? This article explores how polio vaccines work, their effectiveness, factors influencing protection, and the importance of continued vaccination efforts.
How Polio Vaccines Work
There are two primary types of polio vaccines that generate an immune response: the Inactivated Poliovirus Vaccine (IPV) and the Oral Poliovirus Vaccine (OPV). IPV, also known as the Salk vaccine, consists of killed poliovirus strains and is administered via injection. This vaccine stimulates antibodies in the bloodstream, which prevent the virus from spreading to the central nervous system, protecting against paralytic disease.
The Oral Poliovirus Vaccine (OPV) contains live, weakened forms of the poliovirus and is given by mouth. OPV induces both a systemic immune response in the blood and a local immune response in the intestines. This intestinal immunity helps limit virus multiplication in the gut, which is the primary site for poliovirus replication and shedding. OPV’s ability to induce mucosal immunity also helps prevent person-to-person spread of the virus.
The Degree of Protection Vaccines Offer
While polio vaccines are highly effective, no vaccine provides 100% protection against infection. The Inactivated Poliovirus Vaccine (IPV) is highly effective in preventing paralytic polio, with two doses offering 90% or more protection, and three doses providing 99% to 100% effectiveness. This strong protection extends to all three types of poliovirus. Breakthrough cases, where a vaccinated individual contracts polio, are rare.
The Oral Poliovirus Vaccine (OPV) also offers high protection against paralytic poliomyelitis, with three doses providing 96% to 98% effectiveness. A key difference between the two vaccines lies in their ability to prevent intestinal infection and viral shedding. While IPV primarily protects against the severe paralytic form, it induces low intestinal immunity, meaning a vaccinated person could still shed the virus. OPV is more effective at preventing intestinal infection and viral shedding, which helps stop community spread.
Factors Affecting Vaccine Protection
Several factors can influence the effectiveness of polio vaccine protection. One factor is incomplete vaccination, where an individual has not received all recommended doses. Full vaccination involves a series of doses, and missing any can leave a person less protected. For example, while two doses of IPV offer substantial protection, three doses provide nearly complete immunity against paralytic polio.
Individuals with compromised immune systems, due to illness or medications, may not mount as strong an immune response as healthy individuals. While IPV is safe for immunocompromised people and can offer some protection, their response might be suboptimal. Oral Poliovirus Vaccine (OPV), being a live vaccine, is generally not recommended for immunocompromised individuals or their household contacts due to the risk of vaccine-associated paralytic polio (VAPP) or vaccine-derived poliovirus (VDPV) transmission.
Vaccine-derived polioviruses (VDPVs) represent another factor that can affect protection, particularly in areas with low vaccination coverage. VDPVs can emerge from the weakened live virus in OPV, which can, in rare instances, mutate and regain the ability to cause paralysis if allowed to circulate in under-immunized populations for extended periods. While VDPVs are rare, they highlight the importance of high vaccination rates to prevent the circulation and evolution of such strains.
The Importance of Continued Vaccination Efforts
Ongoing vaccination efforts are crucial for global public health, extending beyond individual protection to safeguard entire communities. High vaccination rates contribute to herd immunity, which occurs when a significant portion of the population is immune to a disease, making its spread unlikely. This collective immunity protects vulnerable individuals who cannot be vaccinated, such as infants or those with compromised immune systems.
The Global Polio Eradication Initiative, launched in 1988, aims to permanently stop the circulation of poliovirus worldwide. This initiative has drastically reduced polio cases globally. Continued vaccination prevents the re-introduction and spread of the virus, especially as wild poliovirus still circulates in a few countries. Maintaining high vaccination coverage ensures that the gains made against polio are not reversed, protecting present and future generations from this debilitating disease.