Infectious Agent

Polioviruses (genus Enterovirus ) are small, nonenveloped viruses with a single-stranded RNA genome. Polioviruses are rapidly inactivated by heat, formaldehyde, chlorine, and ultraviolet light. There are 4 poliovirus serotypes, 1, 2 and 3, that have minimal heterotypic immunity between them.


The virus enters through the mouth and multiplies in the throat and gastrointestinal tract. Virus may be excreted in nasopharyngeal secretions for 1–2 weeks and in stools 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.


Before a vaccine was available, infection with wild poliovirus (WPV) was common worldwide, with seasonal peaks and epidemics in the summer and fall in temperate areas. The incidence of poliomyelitis in the United States declined rapidly after the licensure of inactivated poliovirus vaccine (IPV) in 1955 and live oral polio 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 upon the success in the Americas, the Global Polio Eradication Initiative (GPEI) was initiated in 1988 and has made great progress in interrupting WPV transmission globally, drastically reducing the number of countries where travelers are at risk for acquiring polio. WPV type 2 was last isolated in 1999 and declared eradicated in 2015; WPV type 3 has not been detected since November 2012. In 2018, WPV type 1 circulates in 3 countries: Afghanistan, Pakistan, and Nigeria.

In spite of progress made in eradicating WPVs globally, some polio-free countries with low vaccination coverage remain at risk for poliomyelitis outbreaks after importation of WPV type 1. Countries that have low OPV coverage in routine immunization are also at risk of experiencing poliomyelitis cases and outbreaks caused by circulating vaccine-derived poliovirus (cVDPVs). The live attenuated poliovirus strains contained in the Sabin OPV may circulate in areas with inadequate OPV coverage and revert to having wild-like characteristics. Rarely, OPV can cause paralytic polio in vaccine recipients or their close contacts (vaccine-associated paralytic poliomyelitis or VAPP).

From 2006 through 2015, more than 90% of cVDPV cases were type 2. Given the eradication of WPV type 2, experts recommended global withdrawal of OPV serotype 2. Approximately 155 OPV-using countries and territories switched from trivalent OPV (tOPV, containing serotypes 1, 2, and 3) to bivalent OPV (bOPV, containing serotypes 1 and 3) in April 2016. During 2017, WPV type 1 was responsible for 26 paralytic cases in Afghanistan and Pakistan, whereas cVDPV type 2 was responsible for 74 cases in Syria and 22 in the Democratic Republic of Congo. The cVDPV causing these large outbreaks emerged from OPV doses provided before April 2016.

Travelers to countries with current or recent WPV or cVDPV outbreaks may 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.

For additional information on the status of polio eradication efforts, countries or areas with active WPV or VDPV circulation, and vaccine recommendations, consult the travel notices on the CDC Travelers’ Health website (www.cdc.gov/travel) and the GPEI website (www.polioeradication.org).

Clinical Presentation

Most poliovirus infections are asymptomatic; about 25% cause minor illness with total recovery. However, about 0.5% of unvaccinated people may develop acute flaccid paralysis. Paralysis may affect 1 or several limbs, and in severe cases it may result in quadriplegia, respiratory failure, and rarely, death. Many people partially recover muscle function, and some recover completely, but worsening of weakness or paralysis may occur 20–30 years later (postpolio syndrome). Adults with poliovirus paralysis have more severe disease and a worse prognosis than children.


Poliovirus is identified in clinical specimens (usually stool) obtained from an acutely ill patient. Shedding in fecal specimens may 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 following paralysis onset, poliovirus can also be detected from oropharyngeal specimens, but stool specimens are the preferred source for diagnosis. Poliovirus is rarely detected in the blood or cerebrospinal fluid.

Poliovirus is detected in specimens by cell culture, which is followed by PCR of isolates to identify the serotype and whether it is WPV, VDPV, or the vaccine (Sabin) strain. Genomic sequencing of poliovirus isolates determines the geographic origin of WPV and the estimated time of circulation since the original OPV dose for VDPV.

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


Only symptomatic treatment is available.



Recommendations for Health Protection

IPV is the only polio vaccine available in the United States since 2000, but bivalent OPV is used in most middle- and low-income countries and for global polio eradication activities. For complete information on recommendations for poliomyelitis vaccination, consult the Advisory Committee on Immunization Practices recommendations website (www.cdc.gov/vaccines/hcp/acip-recs/vacc-specific/polio.html) and the World Health Organization position paper on poliovirus vaccines (www.who.int/wer/2016/wer9112/en/).

Before traveling to areas that have WPV or VDPV circulation, travelers should ensure that they have completed the recommended age-appropriate polio vaccine series (see Infants and Children below). Adults who have completed a primary series should receive a single lifetime IPV booster dose if traveling to these areas. CDC also recommends a single lifetime IPV booster dose for adult travelers to some countries that border areas with WPV circulation based on evidence of historical cross-border transmission. These recommendations apply only to travelers to bordering countries with a high risk of exposure to someone with imported WPV infection, such as those 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 during the previous 12 months of poliovirus circulation (endemic WPV, active WPV or cVDPV outbreaks, or environmental isolation [sewage sampling]). Since the situation is dynamic, refer to the CDC Travelers’ Health website destination pages for the most up-to-date polio vaccine recommendations (wwwnc.cdc.gov/travel/destinations/list).

Country Requirements

In May 2014, the Director General of the World Health Organization (WHO) declared the international spread of polio to be a public health emergency of international concern under the authority of the International Health Regulations (2005). To prevent further spread of disease, WHO issued temporary polio vaccine recommendations for long-term travelers (staying >4 weeks) and residents departing from countries with WPV transmission (“exporting WPV” or “infected with WPV”). The IHR emergency committee on polio meets every 3 months and updates these recommendations (available at www.who.int/mediacentre/news/statements). In November 2015, these recommendations were extended to long-term travelers (staying >4 weeks) and residents departing from countries with VDPV transmission (“infected with VDPV”).

Clinicians should be aware that long-term travelers and residents may be required to show proof of polio vaccination when departing from these countries. All polio vaccination administration should be documented on an International Certificate of Vaccination or Prophylaxis (ICVP). The polio vaccine must be received between 4 weeks and 12 months before the date of departure from the polio-affected country. Country requirements may change, so clinicians should check for updates on the CDC Travelers’ Health website (wwwnc.cdc.gov/travel/news-announcements/polio-guidance-new-requirements) for a list of affected countries, guidance on meeting the vaccination requirements, and instructions on how to order and fill out the ICVP.

Infants and Children

In the United States, all infants and children should receive 4 doses of IPV, at ages 2, 4, and 6–18 months and 4–6 years. The final dose should be administered at age ≥4 years, 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 age ≥4 years and ≥6 months after the previous dose. If the routine series cannot be administered within the recommended intervals before protection is needed, the following alternatives are recommended:

  • The first dose should be given to infants at age ≥6 weeks.
  • The second and third doses should be administered ≥4 weeks after the previous doses.
  • The minimum interval between the third and fourth doses is 6 months.

If the age-appropriate series is not completed before departure, the remaining IPV doses to complete a full series should be administered when feasible, at the intervals recommended above.


Adults who are traveling to areas where WPV or VDPV is actively circulating and who are unvaccinated, incompletely vaccinated, or whose vaccination status is unknown should receive a series of 3 doses: 2 doses of IPV administered at an interval of 4–8 weeks; a third dose should be administered 6–12 months after the second. If 3 doses of IPV cannot be administered within the recommended intervals before protection is needed, the alternatives are shown in Table 4-15.

Table 4-15. Alternative adult polio vaccine regimens

Time before travel

Number of doses 1

Interval between doses

<4 weeks


Not Applicable

4–8 weeks


4 weeks

>8 weeks


4 weeks

1 If <3 doses are administered, the remaining doses required to complete a 3-dose series should be administered when feasible, at the recommended intervals above, if the person remains at risk for poliovirus exposure.

Guidance for Children Who Have 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 acceptable as evidence of previous vaccination. Please see www.cdc.gov/mmwr/volumes/66/wr/mm6601a6.htm and its erratum www.cdc.gov/mmwr/volumes/66/wr/mm6606a7.htm?s_cid=mm6606a7_w for information on interpreting international poliovirus vaccination documentation.

Children with full vaccination status, who have only received bOPV in the primary series because they were born after April 2016, or a combination of tOPV and bOPV, should be revaccinated with a full IPV series to ensure protection against all 3 poliovirus types. Children <18 years of age without adequate documentation of poliovirus vaccination should be vaccinated or revaccinated in accordance with the age-appropriate US IPV schedule.

The international adoption of children from countries or areas where WPV or VDPV is actively circulating is a special situation. International adoptees might not have completed a primary vaccination series against polio nor received a dose of polio vaccine before departure. Thus, there is a small risk that 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 whose origin is a country with active WPV or VDPV circulation should be assessed before the entry of the child into the United States. Those who are unvaccinated, incompletely vaccinated, or whose vaccination status is unknown should be brought up-to-date.

Vaccine Safety and Adverse Reactions

Minor local reactions (pain and redness) can occur after IPV administration. IPV should not be administered to people who have experienced a severe allergic reaction (such as anaphylaxis) after a previous dose of IPV. Because IPV contains trace amounts of streptomycin, polymyxin B, or neomycin, hypersensitivity reactions can occur after IPV administration among people allergic to these antibiotics.

Pregnancy and Breastfeeding

If a pregnant woman 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. Breastfeeding is not a contraindication to administration of polio vaccine to an infant or mother.

Precautions and Contraindications

IPV may 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 may be administered safely to immunocompromised travelers and their household contacts. Although a protective immune response cannot be ensured, IPV might confer some protection to the immunocompromised person. People 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.

CDC website: www.cdc.gov/polio


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Concepción F. Estívariz, Janell Routh, Manisha Patel, Steven G. F. Wassilak