Polio vaccine

A polio vaccine is a medical tool used to prevent poliomyelitis, commonly known as polio. This disease is caused by a virus that can lead to serious health issues. Vaccines work by training the human immune system to recognize and fight the virus. There are two primary types of vaccines used in global health efforts today. One is the inactivated poliovirus vaccine, or IPV, which is administered via injection. The other is the oral poliovirus vaccine, or OPV, which is given by mouth.

Both types are essential tools for the World Health Organization (WHO) in its goal to eradicate the disease.

The IPV works by using a killed version of the virus. This version is grown in a type of monkey kidney tissue culture called a Vero cell line. Scientists then use formalin to inactivate the virus so it cannot cause disease. When a person receives the injection, their body develops IgG-mediated immunity in the bloodstream. This prevents the virus from progressing to the nervous system. After two doses, 90% or more of people develop protective antibodies to all three poliovirus serotypes. Following three doses, at least 99% of recipients are immune.

The OPV works differently because it uses a live-attenuated virus. This means the virus is alive but has been weakened so it does not cause serious illness. Because it is given by mouth, it is very easy to administer during mass vaccination campaigns. It does not require sterile syringes or extensive medical training. OPV is particularly effective because it creates immunity in the intestines. This is the primary site where the wild poliovirus enters the body. One dose of trivalent OPV produces immunity in about 50% of recipients. Three doses can produce protective antibodies in more than 95% of people.

History shows a progression of different vaccine technologies. In 1950, Hilary Koprowski demonstrated the first successful polio vaccine. He used a live attenuated virus that people drank. While it was not approved in the United States, it was used successfully in other places. In 1955, Jonas Salk announced the success of his inactivated vaccine. Later, in 1961, Albert Sabin’s live oral vaccine came into commercial use.

These different approaches provided scientists with various ways to fight the virus globally.

The impact of these vaccines on global health is massive. In 1988, there were an estimated 350,000 reported cases of polio each year. By 2018, that number had dropped to just 33 cases.

This reduction is due to the interruption of person-to-person transmission. Poliovirus has no non-primate reservoir in nature, meaning it only lives in humans. Additionally, there is no long-term carrier state for the virus in people with normal immune function. This makes it possible to stop the virus entirely through high vaccination coverage.

However, the OPV carries a small, inherent risk. Because it uses a live virus, it can cause vaccine-associated paralytic poliomyelitis, or VAPP. This occurs in roughly one individual per every 2.7 million doses. In under-vaccinated populations, the vaccine virus can circulate and revert to a neurovirulent form. This is called circulating vaccine-derived poliovirus, or cVDPV. In 2017, cVDPV cases actually outnumbered wild polio cases for the first time.

To solve this, scientists developed the novel oral polio vaccine type 2, or nOPV2. This new vaccine is genetically modified to be more stable and less likely to cause outbreaks.

Vaccination schedules vary depending on the risk of polio in a specific country. In areas where polio is still present, the WHO recommends a mix of OPV and IPV. This often starts with OPV at birth, followed by a primary series of three OPV doses. In countries with very high immunization coverage, the focus shifts more toward IPV. For the highest-risk areas, a primary series of three IPV injections is recommended.

These complex schedules ensure that different populations receive the specific protection they need to stay safe.