Author(s): Concepción Estívariz, Janell Routh, Steven Wassilak

Infectious Agent

Polioviruses (genus Enterovirus) are small, nonenveloped viruses with a single-stranded RNA genome. Polioviruses are rapidly inactivated by chlorine, formaldehyde, heat, and ultraviolet light. Poliovirus has 3 serotypes, 1, 2, and 3, that evoke minimal heterotypic immunity between them.

Transmission

Transmission occurs when the virus enters through the mouth and multiplies in the throat and gastrointestinal tract. Virus can be excreted in nasopharyngeal secretions for 1–2 weeks and in stool for 3–6 weeks, even in people who develop no symptoms after infection. Transmission occurs from person to person through the oral and fecal–oral routes.

Epidemiology

Before a vaccine was available, infection with wild poliovirus (WPV) was common worldwide and had seasonal peaks and epidemics in the summer and fall in temperate areas. The incidence of poliomyelitis (polio) in the United States declined rapidly after the licensure of inactivated poliovirus vaccine (IPV) in 1955 and live oral poliovirus vaccine (OPV) in the 1960s. The last cases of indigenously acquired polio in the United States occurred in 1979 and in the Americas in 1991.

Built on the success achieved in the Americas, the Global Polio Eradication Initiative (GPEI) began in 1988 and has made great progress in interrupting WPV transmission globally. Type 2 WPV was last isolated in 1999 and was declared eradicated in 2015; type 3 WPV was last detected in November 2012 and was declared eradicated in 2019. In 2021, type 1 WPV endemic circulation persisted in only 2 countries, Afghanistan and Pakistan. Despite achievements in eradicating WPV globally, polio-free countries with low vaccination coverage remain at risk for poliomyelitis outbreaks after importation of WPV. In February 2022, for example, a person in Malawi (onset of paralysis, November 2021) was identified with type 1 WPV, 18 months after Africa was certified free of indigenous WPV (in August 2020). The virus isolated from this patient was genetically linked to a type 1 WPV lineage last detected in Pakistan in 2019.

Countries that have low OPV coverage in routine immunization also are at risk of experiencing poliomyelitis cases and outbreaks caused by circulating vaccine-derived poliovirus (cVDPV). The live attenuated poliovirus strains contained in the Sabin OPV can circulate in areas with inadequate OPV coverage and revert to having wild-like characteristics. Rarely, OPV can cause paralytic poliomyelitis in vaccine recipients or their close contacts, known as vaccine-associated paralytic poliomyelitis (VAPP).

Because >90% of cVDPV cases detected between 2006 and 2014 were serotype 2, all OPV-using countries conducted a synchronized switch from trivalent OPV (tOPV, containing serotypes 1, 2, and 3) to bivalent OPV (bOPV, containing serotypes 1 and 3) in April 2016. Despite this effort, during 2019–2020, serotype 2 cVDPV caused 1,445 cases of poliomyelitis in 26 countries, whereas serotype 1 cVDPV caused 46 cases in 5 countries, and WPV caused 316 cases in 2 countries. In 2021, cVDPV serotypes 1 or 2 were isolated from 659 cases of poliomyelitis in 23 countries (data as of April 5, 2022).

Travelers to countries with current or recent WPV or cVDPV outbreaks can be at risk for exposure to poliovirus. The last documented case of WPV-associated paralysis in a US resident traveling abroad occurred in 1986 in a 29-year-old vaccinated adult who had been traveling in South and Southeast Asia. In 2005, an unvaccinated US adult traveling abroad acquired VAPP after contact with an infant recently vaccinated with OPV. Of special concern are people with underlying primary immunodeficiencies that prevent an adequate antibody response to viruses; they are at a greater risk for prolonged poliovirus infection and paralysis from WPV, cVDPV, or from exposure to people recently vaccinated with OPV.

For additional information on the status of polio eradication efforts, countries or areas with active WPV or VDPV circulation, and vaccine recommendations, consult the GPEI website and the polio travel health notices on the Centers for Disease Control and Prevention (CDC)’s Travelers’ Health website.

Clinical Presentation

Most poliovirus infections are asymptomatic; ≈25% cause minor illness with full recovery. Depending on poliovirus type, ≈1 in 200 to ≈1 in 2000 infections are associated with paralysis. Paralysis affects ≥1 limbs, and in severe cases can result in quadriplegia, respiratory failure, and rarely, death. Residual paralysis occurs in ≈2/3 cases, and for many people with residual deficits, and even for some who recover fully, worsening of weakness or paralysis can occur 20–30 years later, known as post-polio syndrome. Adults who develop paralysis usually have more severe disease and a worse prognosis than children.

Diagnosis

Information on diagnostic testing for poliovirus is available from CDC’s Polio Laboratory Testing website  and Test Directory: Submitting Specimens to CDC.

Poliovirus can be detected in clinical specimens (usually stool) obtained from an acutely ill patient. Shedding in fecal specimens can be intermittent and declines over time, but poliovirus can be detected for up to 60 days after onset of paralysis. During the first 3–10 days after paralysis onset, poliovirus also can be detected from oropharyngeal specimens, but stool specimens are the preferred source for diagnosis. Poliovirus rarely can be detected in the blood or cerebrospinal fluid; in most cases, blood antibody titers are not useful for diagnosis.

Poliovirus is detected by virus isolation in cultured cells. PCR testing of poliovirus isolates can identify the serotype and whether it is WPV, VDPV, or the vaccine (Sabin) strain. Genomic sequencing of poliovirus isolates can determine the geographic origin of WPV and the estimated time of circulation since the original OPV dose for VDPV.

Paralytic polio is designated an immediately notifiable, extremely urgent disease, which requires state and local health authorities to notify CDC within ≤4 hours of identification. Because of new safety requirements in handling polioviruses, CDC is the only laboratory in the United States permitted to test specimens from a suspected paralytic polio case. Notify CDC through the Emergency Operations Center (EOC, 770-488-7100) or through state health authorities. CDC’s EOC will connect callers with polio subject matter experts who can provide consultation regarding the collection of clinical specimens and procedures.

Treatment

No licensed treatment exists for poliovirus infection, and only supportive care for symptoms is available. Two antiviral agents are undergoing clinical testing for the treatment of people with immunodeficiencies who are infected with and excreting VDPV.

Prevention

Vaccine

Health Protection Recommendations

Since 2000, IPV is the only polio vaccine available in the United States, but bivalent OPV is used in most low- and middle-income countries for routine immunization series and for global polio eradication activities. In response to cVDPV serotype 2 outbreaks, the population should be immunized with genetically stabilized novel OPV2, monovalent OPV2, or trivalent OPV when serotype co-circulation occurs, as in Afghanistan and Pakistan. For complete information on recommendations for poliomyelitis vaccination, consult the Advisory Committee on Immunization Practices (ACIP) website and the March 2016 World Health Organization (WHO) position paper on poliovirus vaccines [PDF].

Before they go to areas where WPV or VDPV is circulating, ensure travelers have completed the recommended age-appropriate polio vaccine series (see Infants & Children, later in this chapter). Adults who have completed a primary series should receive a single lifetime IPV booster dose if traveling to those areas. CDC also recommends a single lifetime IPV booster dose for adult travelers going to some countries that border areas with WPV circulation based on evidence of historical cross-border transmission. These recommendations apply only to travelers going to bordering countries with a high risk for exposure to someone with imported WPV infection (e.g., people who will be working in health care settings, refugee camps, or other humanitarian aid settings).

Countries are considered to have WPV or VDPV circulation if they have evidence of poliovirus circulation during the previous 12 months, either endemic circulation of WPV, active WPV, or cVDPV outbreaks, or environmental isolation through sewage sampling. Because poliovirus circulation is dynamic, refer to CDC’s Travelers’ Health website destination pages for the most up-to-date polio vaccine recommendations by country.

Country Requirements

In May 2014, the Director General of the WHO declared the international spread of poliovirus to be a public health emergency of international concern under the authority of the International Health Regulations (IHR). To prevent further spread of virus, WHO issued temporary polio vaccine recommendations for travelers staying >4 weeks and residents departing from countries with risk for poliovirus spread. The IHR emergency committee on polio meets every 3 months and updates the list of countries that must continue the temporary polio vaccine recommendations to reduce the risk for international spread of poliovirus. See updated IHR reports.

The polio vaccine must be received 4 weeks to 12 months before the date of departure from the polio-affected country. Be aware that long-term travelers and residents might be required to show proof of polio vaccination when departing from these countries, and document all polio vaccination administration on an International Certificate of Vaccination or Prophylaxis (ICVP). Country requirements might change, so check for updates on the CDC Travelers’ Health website for a list of affected countries and guidance on meeting the vaccination requirements. For ordering information and instructions on how to fill out the ICVP.

Adults

Before traveling to areas where WPV or VDPV is circulating, adults who are unvaccinated, incompletely vaccinated, or whose vaccination status is unknown should receive a series of 3 doses: 2 doses of IPV administered 4–8 weeks apart, and a third dose administered 6–12 months after the second dose. If 3 doses of IPV cannot be administered within the recommended intervals, alternative dosing schedules are available (see Table 5-16).

Immunocompromised People

IPV can be safely administered to immunocompromised travelers and their household contacts. Although a protective immune response cannot be ensured, IPV might confer some protection to people who are immunocompromised. Those with certain primary immunodeficiency diseases should not be given OPV and should avoid contact with children vaccinated with OPV overseas in the previous 6 weeks.

Infants & Children

In the United States, all infants and children should receive 4 doses of IPV, given at ages 2, 4, and 6–18 months, and 4–6 years. The final dose should be administered at ≥4 years of age, regardless of the number of previous doses, and should be given ≥6 months after the previous dose. A fourth dose in the routine IPV series is not necessary if the third dose was administered at ≥4 years of age and ≥6 months after the previous dose. If the routine series cannot be administered within the recommended intervals before protection is needed, CDC recommends the following alternative: give the first dose to infants at ≥6 weeks of age, give the second ≥4 weeks after dose 1, and the third dose ≥4 weeks after dose 2, then give the fourth dose ≥6 months after dose 3.

If the age-appropriate series is not completed before international travel, administer the remaining IPV doses to complete a full series when feasible, at the intervals recommended above. In addition, children completing the accelerated schedule should still receive a dose of IPV at ≥4 years of age, provided ≥6 months have passed since the last dose.

Children Who Received Poliovirus Vaccine Outside the United States

Vaccines administered outside the United States generally can be accepted as valid doses if the schedule is similar to that recommended in the United States. Vaccination against polio is also valid for children from countries that use an accelerated schedule. Only written, dated records are accepted as evidence of previous vaccination. Please see Guidance for Assessment of Poliovirus Vaccination Status and Vaccination of Children Who Have Received Poliovirus Vaccine Outside the United States and its erratum for information on interpreting international poliovirus vaccination documentation.

Children with full vaccination status who received only bOPV in the primary series because they were born after April 2016, or who received a combination of tOPV and bOPV, should be revaccinated with a full IPV series to ensure protection against all 3 poliovirus types. If the child has documentation of receipt of an IPV dose, this can be considered the first dose in the US dosing schedule. In accordance with the age-appropriate US IPV schedule, vaccinate or revaccinate children <18 years old without adequate documentation of poliovirus vaccination.

International adoption from countries or areas where WPV or VDPV is actively circulating is a special situation. International adoptees might not have completed a primary polio vaccination series nor received a polio vaccine dose before departure. Thus, although the risk is small, they could be infected with WPV or VDPV and remain infectious upon entry into the United States, and potentially transmit to US household members and caregivers. As a measure of prudence, the polio vaccination status of all household members and caregivers of international adoptees from a country with active WPV or VDPV circulation should be assessed before the child enters the United States (see Sec. 7, Ch. 5, International Adoption). People who are unvaccinated, incompletely vaccinated, or whose vaccination status is unknown should be brought up to date.

Pregnancy & Breastfeeding

If a pregnant person is unvaccinated or incompletely vaccinated and requires immediate protection against polio because of planned travel to a country or area where WPV or VDPV is actively circulating, IPV can be administered as recommended for adults. In addition, neither is breastfeeding a contraindication to receiving polio vaccine.

Precautions & Contraindications

IPV can be administered to people with diarrhea. Minor upper respiratory illnesses with or without fever, mild to moderate local reactions to a previous dose of IPV, current antimicrobial therapy, and the convalescent phase of acute illness are not contraindications to vaccination. IPV can be coadministered with other vaccines.

Safety & Adverse Reactions

Minor local reactions (pain and redness) can occur after IPV administration. Do not administer IPV to people who have experienced a severe allergic reaction (e.g., anaphylaxis) after a previous dose of the vaccine. Because IPV contains trace amounts of neomycin, polymyxin B, or streptomycin, hypersensitivity reactions can occur after IPV administration among people allergic to these antibiotics.

CDC website: Polio