Thanks to widespread vaccination efforts, many diseases that were once common, particularly during childhood, have faded from memory. Polio (also known as poliomyelitis) is a perfect example. As The History of Vaccines explains, periodic epidemics of this highly contagious and potentially fatal disease first began in the United States in 1894.
By the late 1940s and early 1950s, yearly summertime outbreaks were common, especially among children. In 1952 alone, an estimated 57,628 individuals contracted the disease. According to the Centers for Disease Control and Prevention (CDC), in the early 1950s, before the development of a vaccine, more than 15,000 children were paralyzed each year by polio In the United States. Cases dropped drastically after the vaccine’s release, falling to below 100 in the 1960s and fewer than 10 in the 1970s. No cases of polio have originated in the United States since 1979.
That is, until July 21, 2022, when an unvaccinated man in Rockland County, New York, was diagnosed with polio. The New York State Department of Health determined the man contracted the disease in the community, and in August, samples of wastewater collected in and around New York City were found to contain poliovirus. This sparked New York governor Kathy Hochul to declare a disaster in the state, freeing up resources to tackle what some fear may become the beginning of a widespread polio outbreak (via the British Medical Journal). But what exactly is polio, why is it considered so dangerous, and are you adequately protected?
As the Merck Manual explains, polio is caused by poliovirus, a type of enterovirus. Enteroviruses are very small (even by virus standards) RNA-containing viruses. There are more than 100 enteroviruses, and they are widely distributed throughout the world. As a group, they’re responsible for many diseases in humans, although most are mild. In some cases, conditions caused by enteroviruses may also be caused by infections with other types of viruses. Enteroviruses multiply in either the intestines or the respiratory tract, and virus particles are shed either in stool or through respiratory secretions.
The Global Polio Eradication Initiative (GPEI) notes that the poliovirus consists of RNA (the genetic material that tells infected host cells to replicate more copies of the virus) encased in a protein shell called a capsid. There are three strains, or serotypes, of poliovirus — types 1, 2, and 3 — each with a slightly different protein shell. These capsids are different enough, however, that infection with one strain of poliovirus doesn’t provide immunity against future infections from the two other strains.
Polio outbreaks were very uncommon in the United States before the 20th century. In the past, poor sanitation meant that children were usually exposed to poliovirus at a very young age — so young, in fact, that they were sill protected by maternal antibodies acquired before birth or during breastfeeding. Improvements in sanitation during the early 1900s meant that children weren’t exposed to poliovirus until later in childhood, when their maternal antibodies had worn off.
Polio is very contagious
As Vaccines Today explains, a virus’s reproductive rate or R0 (pronounced "R naught") indicates how easily the virus spreads by estimating how many people one sick person will infect. Polio has an R0 of between 4 and 6, meaning that on average, one individual with polio will give it to four to six other people. For comparison, the R0 for smallpox, a disease that killed tens of millions of people through history, was only 3, while chickenpox has an R0 of 10 to 12 and measles has an R0 of 16 to 18.
Polio’s contagiousness is largely due to the fact that, according to the Centers for Disease Control and Prevention (CDC), many people with the virus never show any symptoms. In addition, the virus is present in respiratory secretions for one to two weeks and will be shed in the stool for several weeks, meaning there’s ample time to inadvertently infect others.
As the Cleveland Clinic states, individuals can get polio by inhaling the virus in respiratory droplets or accidentally ingesting fecal matter from an infected. In addition to close personal contact with someone who has polio, individuals can get the disease from drinking or swimming in water contaminated with the feces of an infected person, eating food that has come into contact with contaminated water, and touching contaminated surfaces. Contamination most often happens when individuals don’t thoroughly wash their hands after going to the bathroom or handling feces (such as changing a dirty diaper).
Who’s most at risk for polio?
Anyone who isn’t vaccinated can get polio, although those under age 5 are most at risk because young children have less than stellar hygiene habits (via the Cleveland Clinic). Other risk factors include living or traveling in an area where polio hasn’t been eliminated and living in poor sanitary conditions. Unvaccinated pregnant women are also at increased risk, as pregnancy stresses the immune system, making individuals more susceptible to infections. According to Children’s Hospital of Philadelphia, although infants and very young children are the most at risk of contracting polio, the risk of paralysis from polio increases with age.
Before polio was eliminated from most of the world, outbreaks of the disease occurred predominantly in the summer and fall in temperate climates like the United States, while in tropical parts of the world, polio didn’t follow this seasonal pattern (via the Centers for Disease Control and Prevention). According to an article published by Elsevier Connect, cases would begin emerging around Memorial Day, peak in August, and then decline by Labor Day. Certain areas would be hit particularly hard one year, and then the next year, outbreaks would be more prevalent in a different area of the country. Because polio could easily spread through contaminated water, beaches and pools would often be closed as soon as cases started appearing. Other public gathering places, such as bowling alleys and movie theaters, would also shut down.
Asymptomatic, abortive, and nonparalytic polio
As dangerous as polio can be, the vast majority of individuals have no or only mild symptoms. According to the Centers for Disease Control and Prevention (CDC), about 70% of poliovirus infections (at least in children) are asymptomatic. This type of infection is also known as inapparent polio (via the University of Rochester Medical Center). The CDC notes, however, that asymptomatic individuals still shed the virus and can thus infect others.
According to the CDC, about 24% of individuals exposed to poliovirus will get a form of the disease known as abortive polio, characterized by mild, nonspecific symptoms and recovery within a week. Per the University of Rochester, these symptoms may include sore throat, fever, decreased appetite, nausea or vomiting, abdominal pain, constipation, and general malaise.
Nonparalytic polio, also known as nonparalytic aseptic meningitis, occurs in 1% to 5% of those with polio (via the CDC). In this version of the disease, an individual initially has symptoms of abortive polio, gets better, and then begins experiencing stiffness in their neck, back, or legs a few days later. Other symptoms include pain in the stiff areas, headache, and vomiting. These symptoms last 2 to 10 days, after which the individual is fully recovered. As the Cleveland Clinic explains, these symptoms are caused by inflammation of the meninges, the protective tissue surrounding the brain and spinal cord. This inflammation may also cause sensitivity to light.
The most dreaded form of polio is paralytic polio, in which the disease spreads to the central nervous system. According to the Centers for Disease Control and Prevention (CDC), paralytic polio occurs in less than 1% of all polio infections in children, although the incidence is slightly higher among adolescents and adults (though still very uncommon). Older individuals also tend to have more severe forms of paralysis. In children, the paralytic form of the disease is usually preceded by abortive or nonparalytic polio, while in adults, the onset of paralysis may be the first symptom they experience.
There are three types of paralytic polio. As the name suggests, spinal paralytic polio involves the spine, leading to weakness or paralysis of the arms or, more commonly, the legs. Among those with paralytic polio, the spinal type accounts for 79% of paralysis cases. Bulbar paralytic polio involves weakness or paralysis of the nerves that control the muscles of the face, throat, and chest, and accounts for 2% of paralysis cases. Bulbospinal paralytic polio is a combination of the two other types and accounts for 19% of paralysis cases. Individuals maintain sensation in the affected areas. In many cases, the damage is permanent.
About 2% to 5% of children with paralytic polio will die, while the morality rate for adolescents and adults with paralysis ranges from 15% to as high as 75%. Not surprisingly, the chance of death is greatest if the muscles that control breathing are affected.
Diagnosing and treating polio
As the Mayo Clinic explains, "doctors often recognize polio by symptoms, such as neck and back stiffness, abnormal reflexes, and difficulty swallowing and breathing." Testing nasal secretions, stool, or the cerebrospinal fluid that surrounds the brain and spinal cord for evidence of poliovirus can confirm the diagnosis.
The University of Rochester Medical Center notes that there’s no cure for polio, so treatments focus on supportive care that addresses specific symptoms. As a result, treatment varies based on the nature and severity of symptoms, as well as an individual’s age and general health. Pain relievers such as ibuprofen or acetaminophen or heating pads may be used to relieve muscle pain, and patients may be advised to eat a healthy diet and minimize activity. For those with paralytic polio, devices to assist with movement, such as leg braces, crutches, canes, or wheelchairs, can improve mobility. Physical and occupational therapy can also help individuals improve their mobility. If the muscles of the chest are affected, supplemental oxygen, ventilators, and other forms of breathing help may be needed.
Individuals may also need to change their diet to make swallowing easier if their throat muscles have been weakened by polio. Although those with asymptomatic, abortive, or nonparalytic polio usually recover fully and may only need supportive care during the duration of the illness, the weakness and paralysis caused by paralytic polio may be permanent and require lifelong use of mobility and breathing aids.
The most iconic treatment for polio before the evolution of modern ventilators was the iron lung. As the Science Museum explains, this device allowed individuals whose respiratory muscles had been paralyzed by polio to breathe. An iron lung was a coffin-like structure attached to bellows that moved air in and out of the chamber. An individual’s entire body would be placed in the iron lung, with just their head outside. A rubber seal around their neck kept the chamber airtight. When the bellows sucked air out of the chamber, the pressure dropped, causing the patient’s lungs to expand and fill with air. When the bellows pumped air back in, the change in air pressure caused the lungs to deflate, forcing air out of the lungs. This form of mechanical breathing assistance is known as external negative pressure ventilation (ENPV).
All care for the person in the iron lung (including cleaning the patient, attending to their bathroom needs, and physical therapy), had to be done by attendants through special portholes in the side of the iron lung or in the few minutes a person might be able to spend outside the device breathing on their own. With just their head exposed, patients (most of whom were young children) could do almost nothing on their own. Fortunately, most people spent only a few weeks or months in iron lungs until their breathing muscles had recovered enough to breathe on their own, but some individuals remained in iron lungs for years.
If you had polio in the past, it’s possible for symptoms to reemerge years later, a condition known as post-polio syndrome (PPS). According to Cedars Sinai, PPS affects up to half of individuals who previously had polio. Symptoms can appear anywhere from 10 to 70 years later, although 20 to 40 years is most common. Symptoms are most likely to affect those parts of the body that were originally affected. The most common signs of PPS include muscle weakness, pain in the muscles and joints, and fatigue. Other symptoms include muscle shrinkage, sleep issues, and sensitivity to cold. Individuals may also have difficulty swallowing or breathing. These symptoms usually get worse over time and can be disabling.
As the Mayo Clinic notes, the cause of PPS is unknown. The most promising hypothesis involves overworking of the nerves. When the initial polio infection damages the motor neurons that control movement, the surviving neurons grow new nerve fibers to compensate. While this speeds up recovery, it also puts a lot of stress on the remaining neurons. Over time, these neurons may not be able to adequately maintain these nerve fibers, which then begin to break down, causing the symptoms of PPS. In support of this theory, those who recovered the most from the initial infection are most likely to get PPS, as are individuals who had a severe case of polio or contracted it after childhood. Individuals with PPS do not shed the virus and aren’t contagious (via the CDC).
Treating post-polio syndrome
As with polio itself, post-polio syndrome (PPS) can’t be cured, and can only be managed to reduce symptoms and improve quality of life. As the British National Health Service (NHS) states, individuals with PPS are usually managed by a team of doctors and specialists. Depending on a person’s symptoms, this team can include a neurologist, respiratory consultant, rehabilitation specialist, physical therapist, occupational therapist, mobility specialist, and speech and language therapist (who can assist with swallowing issues).
Exercise is thought to slow the progression of PPS, so individuals are usually encouraged to remain active while still respecting their physical limits. Painkillers used to treat the pain associated with PPS include over-the-counter options like aspirin or ibuprofen, prescription-strength NSAIDs, gabapentin, and opiates (although these are less likely to be prescribed because of the risk of addiction and abuse). Assistive devices such as walking sticks, mobility scooters, or CPAP machines can make moving and breathing easier, as can following specific rehabilitation exercises. Because excess weight puts additional strain on already weakened muscles, people with PPS are encouraged to maintain a healthy weight through diet and exercise. Because of its often progressive nature and ability to limit daily activity, PPS can take an enormous psychological toll on individuals. It may also bring back painful childhood memories surrounding the original experience with polio. People with PPS may therefore also need to seek treatment for depression, including therapy or medication.
Development of the polio vaccine
A major victory in the fight against polio came with the development of effective vaccines. As the Science History Institute explains, Jonas Salk developed the first polio vaccine, which was approved in 1955. This vaccine contained inactivated (killed) virus and was injected. Salk had pioneered the development of inactivated vaccines years earlier when he developed an influenza vaccine that used killed virus particles. But Salk’s creation of a killed polio vaccine wouldn’t have been possible if it weren’t for the 1949 discovery by John Enders, Thomas Weller, and Frederick Robbins that poliovirus could be grown in laboratory cultures of non-nerve tissue. This made it possible to grow the large numbers of virus needed to create vaccines for widespread use. Once clinical trials determined the vaccine was safe and effective, a number of pharmaceutical companies, including Eli Lilly and Company, Wyeth Laboratories, and Parke, Davis and Company began making the vaccine, and a nationwide vaccination campaign began.
An oral polio vaccine developed by Albert Sabin that used live attenuated (weakened) virus became available in 1961 after widespread testing in the Soviet Union. In the United States, Pfizer made Sabin’s live vaccine. Although vaccination efforts had widespread and enthusiastic public support, there was controversy over which vaccine was best. Research showed that in very rare cases, Sabin’s live attenuated vaccine could cause polio with paralysis, so in 1999, Salk’s inactivated vaccine became the preferred option in the United States (although Sabin’s was used extensively around the globe).
Although polio has been largely eliminated from the United States, vaccination against the disease is still part of the recommended immunization schedule for children. As the Centers for Disease Control and Prevention (CDC) notes, the inactivated polio vaccine (IPV) is given as a four-dose series. The first dose is recommended at 2 months of age, and should not be given before 6 weeks of age. The second dose is given at 4 months, while a third dose should be administered sometime between 6 and 18 months. A final dose is given between ages 4 and 6. If parents have fallen behind with this vaccination schedule for their children, it’s possible to get "catch up" vaccinations, as long as the proper spacing between doses is followed. Although the oral live attenuated polio vaccine (OPV) isn’t usually given, it follows the same schedule as the IPV. Caution should be taken when administering the polio vaccine to a child who has an acute illness.
Generally, adults who were fully vaccinated as children don’t need a booster unless they’re considered to be at high risk, in which case a single booster of IPV is given and lasts a lifetime. High-risk situations include traveling to areas of the world where polio still exists and working in a lab or healthcare facility where you handle polio specimens or care for individuals with polio. For adults who were never vaccinated, a three-dose series of IPV is sufficient (via the CDC).
Polio vaccine concerns
Although the polio vaccine is considered safe and effective, some issues can arise. As with any medication, it’s possible to have an allergic reaction to the polio vaccine. Per the Mayo Clinic, the inactivated polio vaccine (the form used in the United States today) contains trace amounts of streptomycin, polymyxin B, and neomycin — antibiotics that some people are allergic to. Signs of allergic reaction after receiving the polio vaccine show up quickly and can include trouble breathing, wheezing, rapid heart rate, hives, dizziness, and weakness.
According to the Cleveland Clinic, in some cases the oral polio vaccine can lead to the spread of polio. Because the oral vaccine uses live but weakened poliovirus and because individuals can shed this weakened virus for a while after receiving the vaccine, it’s possible for the shed weakened virus to mutate over time and eventually become strong enough to infect unvaccinated individuals. This is known as circulating vaccine-derived poliovirus (cVDPV). But cVDPV can only spread in areas where few people are unvaccinated, as those who’ve been vaccinated are immune to cVDPV. Although the oral vaccine has been instrumental in worldwide vaccination campaigns, the inactivated vaccine is preferred because it cannot cause cVDPV. According to the Centers for Disease Control and Prevention (CDC), only a handful of cases of cVDPV have been reported in the United States. cVDPV is responsible for the polio case in New York detected in July 2022 (via the Center for Infectious Disease Research and Policy).
Eliminating polio worldwide
As the Centers for Disease Control and Prevention (CDC) explains, the fear that polio caused led to widespread support of vaccination efforts in the United States. Just a couple decades after the development of both the inactivated and live attenuated polio vaccines, polio was declared eradicated in the United States. Since 1979, no cases of wild polio (that is, polio not caused by mutation of weakened virus used in the oral vaccine) have been reported in the United States.
According to the World Health Organization (WHO), the global campaign to eradicate polio began in 1988 with the launch of the Global Polio Eradication Initiative. This program was supported by national governments, WHO, Rotary International, the CDC, UNICEF, and, later, the Bill and Melinda Gates Foundation and Gavi, the Vaccine Alliance. Although polio has not been completely eradicated from the world, the Global Polio Eradication Initiative has been hugely successful. As WHO notes, "wild poliovirus cases have decreased by over 99% since 1988, from an estimated 350,000 cases in more than 125 endemic countries then to 175 reported cases in 2019."
Two of the three wild polio strains have been officially certified as globally eradicated, with the last case of polio caused by wild type 2 poliovirus reported in 1999 and the last case of polio caused by wild type 3 poliovirus reported in 2012. Today, wild type 1 poliovirus only circulated in two countries: Pakistan and Afghanistan.