Poliomyelitis, commonly called polio, is a highly infectious disease, caused by the poliomyelitis virus. The vast majority of poliovirus infections do not produce symptoms, but 5 to 10 out of 100 people infected with polio may have some flu-like symptoms. In 1 in 200 cases, the virus destroys parts of the nervous system, causing permanent paralysis in the legs or arms. Although very rare, the virus can attack the parts of the brain that help you breathe, which can cause death.

The virus is usually transmitted from person to person, from the feces of an infected individual, that reach the mouth of another susceptible individual. In some cases, the source of the infection may be water or food contaminated with feces from infected people. Transmission from respiratory secretions has also been described. Because a large proportion of infected people do not have symptoms, the hygiene measures necessary to prevent transmission may not be taken into account with the required importance.

People under the age of 15 who are not vaccinated against polio are at increased risk of polio; among them, those under the age of five are more susceptible. This risk increases when, in places where this person lives or visits, there are others not vaccinated against polio. In these environments, poor hygiene conditions further increase the chances of becoming infected with the poliovirus.

Polio should be suspected in unvaccinated or partially vaccinated children against polio with flu-like symptomatology (fever, muscle aches, headaches, lack of appetite), and who appear to recover and feel better; but after 2 to 5 days afterward they develop a headache, fever, severe muscle aches, involuntary muscle movements and tingling sensation in the legs or arms. One to two days later, the decrease in strength in the legs or arms appears, and the difficulty of walking. The decrease in strength progresses rapidly to paralysis, which is usually uneven among affected members.

Polio can cause permanent paralysis. Cases have been reported of individuals who fully recovered, but 15 to 40 years later developed postpolio syndrome, in which muscle aches, limb weakness and new episodes of paralysis occur again. It has been reported that 2 to 10 out of 100 children with paralytic polio die because paralysis affects the muscles that help to breathe.

To identify the poliovirus, a stool sample from each probable case should be tested in the laboratory, the sample should be collected within 14 days of the onset of paralysis. In the lab, the sample is inoculated in cell cultures where the virus can infect and replicate. This isolated virus is subsequently typified by molecular assays and genetic sequencing tests.

There are many infectious and non-infectious diseases that can cause paralysis, and therefore be mistaken for polio, including Guillaín-Barré syndrome, bumps or traumas of nerves in the spine or major nerves, and spinal tumors. Doctors will run various tests to rule out these diseases that may be mistaken for polio.

There is no antiviral or medicine that stops the progression of paralysis.

The best way to avoid infection is by vaccinating against polio. This vaccination is intended for children under 5 years of age. It is best to complete the vaccination schedule recommended by your country's health authorities in a timely manner. For a child to gain poliovirus immunity, several doses of the vaccine are required.

Two types of polio vaccines are used in the Region of the Americas: oral attenuated vaccine (OPV) and injected inactivated vaccine (IPV). The OPV vaccine contained all three types of poliovirus, 1, 2 and 3. Following the declaration of eradication of wild poliovirus type 2, in 2016, serotype 2 was removed from the vaccine and only the OPV vaccine with serotypes 1 and 3, known as bOPV, continued to be used. Gradually, countries are replacing the bOPV vaccine with the IPV vaccine.

Yes, polio vaccines are very safe and effective in protecting against polio. 

Most people vaccinated against polio have no side effects. The vaccine injected alone has in very rare cases caused mild redness or temporary induration at the site of administration. Allergic reactions associated with vaccines have also been reported.

The oral polio vaccine contains live viruses that have been attenuated to remove the ability to cause paralysis. In children vaccinated with the oral vaccine, attenuated viruses reproduce and persist in their intestines for up to six weeks, and are then excreted into the environment. These vaccine viruses can reach other susceptible children and provide protection. This is known as herd immunity. However, in places where polio vaccination coverage is low, these viruses can often be transmitted among unvaccinated, or partially vaccinated, children. In very rare cases these multiple transmissions can cause viruses to mute or change their genetic characteristics and can regain their ability to produce paralysis. This allows the occurrence of cases with paralysis derived from the polio vaccine.

In people with some primary impairments of their immunity, who have received attenuated poliovirus from the oral vaccine, there is a risk that poliovirus will continue to excrete for long periods of time, implying that the virus continues to replicate or multiply in the intestines. This can cause changes or mutations in viruses. In very rare cases this has allowed them to regain the ability to produce paralysis. Although no secondary cases of paralytic polio have been identified to people with primary immunodeficiencies, due to the excretion of poliovirus with genetic mutations, there is a risk of spread in susceptible communities, which could lead to a paralytic polio outbreak.

Because the inactivated polio vaccine contains dead virus segments, these viruses do not have the ability to replicate or multiply in the individual, so it has not been associated with cases of paralytic polio derived from the vaccine.

The best way to prevent polio is through vaccination. The inactivated vaccine, while protecting the recipient, cannot reproduce in the intestine. This prevents the obtaining of intestinal immunity, which is provided by the attenuated oral vaccine. This intestinal immunity helps prevent wild polioviruses from spreading. On the other hand, intestinal reproduction facilitates herd immunity, which occurs when an attenuated poliovirus is transmitted from a vaccinated person to a susceptible person. Other advantages of the oral vaccine against the inactivated vaccine are its easy application and low cost. These prerogatives lead to the decision to use the oral vaccine in public health programs, generating population immunity and cutting off the transmission of wild poliovirus in infected communities.