Polio and Vaccine-Associated Paralytic Poliomyelitis

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Polio is an ancient disease. Although the first modern polio epidemic is thought to have occurred in 1887, when 44 cases were reported in Stockholm, Sweden, polio likely existed as far back as 1580 B.C.

A type of enterovirus, polio usually causes infections without symptoms or very mild symptoms, including a low-grade fever and sore throat.

Other children can develop more worrisome polio symptoms, though, including those with:

  • Nonparalytic aseptic meningitis — This occurs in 1% to 5% of polio infections. It involves a low-grade fever and sore throat with stiffness of the neck, back, and/or legs, and increased or abnormal sensations, which can last for 2 to 10 days.
  • Paralytic polio — Paralytic polio occurs in less than 1% of cases. Typically children will have mild symptoms at first, then be symptom-free for a few days, and then develop muscle pain, fever, and flaccid paralysis. Roughly two thirds of people with paralytic polio develop permanent weakness and paralysis. Paralytic polio is fatal in at least 2% to 5% of child cases and 15% to 30% of adolescent or adult cases.

Polio hit its peak in the United States in 1952, when there were over 21,000 cases of paralytic polio.

The United States officially eliminated polio in 1979. That last outbreak of locally acquired paralytic polio had been among an unvaccinated group of Amish in several states in the Midwest. Travelers could still acquire polio abroad, but the virus has not been brought into the country since 1993.

Polio Vaccines

Of course, it was the development of the first polio vaccines that stopped the polio epidemics after the 1950s and helped us eliminate the endemic spread of polio.

The Salk vaccine, an inactivated polio vaccine, was licensed in 1955. This was followed by the introduction of the original Sabin vaccine, an oral, live polio vaccine, in 1961.

Both types of polio vaccines have their strengths and weaknesses:

  • The oral polio vaccine (OPV) provides lifelong immunity against polio, including intestinal immunity, but the vaccine can also rarely cause vaccine-associated paralytic polio (VAPP) and vaccine-derived polio.
  • The inactivated polio vaccine (IPV) provides great protection against polio after three doses, especially paralytic polio (intestinal immunity is not as good though), and since it isn't a live virus vaccine, it cannot cause vaccine-associated paralytic polio nor vaccine-derived polio.

When a trivalent oral polio vaccine (protected against all three strains of the polio virus) was introduced in 1963, it replaced the original Sabin vaccines, which only protected against one strain each. The trivalent OPV became the most widely used vaccine in the U.S for decades.

An enhanced version of the Salk vaccine was introduced in 1997, and it went on to replace the oral polio vaccine in many developed countries that had eliminated polio because of concerns about vaccine-associated paralytic polio (VAPP).

When you look at the strengths of the oral polio vaccine, though, it is easy to see why it is used when you are still trying to get wild polio under control in an area. In general, the oral polio vaccine is also less expensive and much easier to give to children, since it doesn't require a shot.

Vaccine-Associated Paralytic Poliomyelitis

Vaccine-associated paralytic poliomyelitis (VAPP) occurs when the weakened live poliovirus strain in the oral polio vaccine changes and causes someone, or a very close contact, to develop symptoms of paralytic polio.

The change occurs in the intestine of someone who has received the oral polio vaccine, typically after the first dose and most commonly in people with immune system problems.

Fortunately, VAPP does not lead to outbreaks of polio, and it is very rare, only occurring in about 1 in every 3 million doses of oral polio vaccine given.

Still, that ended up as an average of eight cases a year in the United States, and once polio was eliminated in the United States, the risk-benefit ratio no longer favored the oral polio vaccine. When the only kids getting polio were getting vaccine-associated paralytic poliomyelitis, it became time to make a switch to the Salk vaccine.

John Salamone became the advocate for that change. His son, David, developed VAPP after getting his oral polio vaccine in 1990. At the time, the live, oral polio vaccine was still a standard part of the childhood immunization schedule.

As early as 1977, an Institute of Medicine report "Evaluation of Poliomyelitis Vaccines" stated that "five major policy options were considered for the United States in the context of the 60-70 percent level of vaccination now reached." These options included using only OPV, only IPV, and a combination of both vaccines, etc. Low vaccination rates seemed to be a big factor in influencing the recommendation to go with only OPV at the time.

As time went by, it became clear that the switch to IPV was necessary, but fear of changing a program that had been working so well for so long and perhaps uncertainty that the switch, including a need to greatly increase the supply of the inactivated vaccine in a short amount of time, kept health experts from making it until 1997.

The sequential IPV/OPV vaccine schedule was formally changed to an all-IPV vaccine schedule in 2000.

Vaccine-Derived Poliovirus

Although it sounds similar to VAPP, vaccine-derived poliovirus strains are a little different.

A vaccine-derived poliovirus (VDPV) strain also undergoes genetic changes from the weakened (attenuated) live poliovirus strain in the oral polio vaccine and can then cause paralytic symptoms, but it also develops the ability to continue circulating and cause outbreaks.

These outbreak or circulating strains of vaccine-derived poliovirus (cVDPV) are fortunately very rare. When they occur, it is because a lot of people in the community aren't vaccinated against polio, as high vaccination rates protect against cVDPV, just like they protect against wild poliovirus strains.

Between 2000 and 2017, there were 24 cVDPV outbreaks resulting in 760 cases. However, it's important to remember that during this time, vaccination prevented over 13 million cases of polio. Furthermore, since 2016 the OPV no longer includes the type 2 component, which had been responsible for 90% of cVDPV cases until that point.

Without polio vaccines, we wouldn't have VAPP, VDPV, and cVDPV, but we would go back to the days when over 200,000 people a year developed paralytic polio.

Post-Polio Syndrome

Post-polio syndrome is another term to be familiar about when studying polio.

Like children who recover from measles and then have a risk of developing subacute sclerosing panencephalitis (SSPE), a post-polio syndrome is a late complication of paralytic polio.

About 25% to 40% of those who had paralytic polio can develop new symptoms 15 to 40 years later. Symptoms of post-polio syndrome can include new muscle pain, new muscle weakness, and even new paralysis. Or they may have worsening of a previous muscle weakness.

Post-polio syndrome does not cause a person to shed the virus. Someone with post-polio syndrome is not contagious.

What You Need to Know about Polio

Other things to know about polio include that:

  • Improved hygiene and sanitation didn't cause polio to disappear, as some anti-vaccine conspiracy theorists argue. Instead, polio changed from an endemic form, infecting most children when they were infants and still had protection from maternal antibodies, to an epidemic form, as fewer people were exposed and developed immunity when they were younger.
  • There are three different serotypes of wild poliovirus (WPV). Natural immunity only provides lifelong immunity to the specific serotype of polio to which you were infected.
  • SV40 contamination in the original polio vaccines from 1955 to 1963 is not associated with an increased risk of cancer.
  • The standard vaccination schedule includes four doses of a polio vaccine at 2 months, 4 months, 6 to 18 months, and a booster dose at 4 to 6 years of age.
  • The Cutter Incident refers to a problem with polio vaccine manufactured by Cutter Laboratories which was not completely inactivated, causing paralytic polio in 260 children and 10 deaths in 1955.
  • Between 1961 and 2020, there have been 149 cases of immunodeficiency-related vaccine-derived poliovirus (iVDPV), in which a person with a rare immune disorder develops paralytic polio after vaccination. The individual may continue to shed poliovirus after vaccination for up to six months. Even though seven of these cases have been known to shed virus for more than five years (all seven cases have common variable immunodeficiency disorder, or CVID), this is not thought to be a common way to spread the polio virus to others.
  • Because of VAPP and VDPV, there will eventually be a worldwide phase-out of the oral polio vaccine and a switch to the inactivated polio vaccine until polio is completely eradicated. Countries are usually not switched to an all-IPV immunization schedule until they demonstrate high vaccination rates and the risk of importation of wild polio is low.
  • In May 2016, the trivalent oral polio vaccine (tOPV) was discontinued globally. Organizations have switched to a bivalent oral polio vaccine (bOPV), removing the type 2 component of the vaccine, decreasing the risk of VAPP and cVDPV.
  • There is no cure for polio.
  • Other conditions that can cause acute flaccid paralysis include non-polio enterovirus infections, rabies, Guillain-Barre syndrome, acute transverse myelitis, and myasthenia gravis. Many other causes of flaccid paralysis also include sensory signs and symptoms though or can be differentiated from polio in other ways.

Most importantly, know that polio is close to being eradicated. In 2019, wild polio only caused 125 cases. Type 1 polio remains endemic in only two countries, Afghanistan and Pakistan. Wild virus type 2 was declared eradicated in 2015, and wild virus type 3 was declared eradicated in 2019.

Get educated. Get vaccinated. Stop the outbreaks.

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