In September 2005, an unvaccinated 7-month-old infant with severe combined immunodeficiency in an Amish community without vaccine coverage for polio was diagnosed with poliovirus by stool sample. The type 1 poliovirus was identified as a vaccine-derived poliovirus from oral poliovirus vaccine (OPV). Three other children in the household had the same virus recovered from stool samples, although the children were not ill. Vaccine-derived poliovirus occurs via replication in an immunocompromised patient or via circulating virus in communities with low vaccine coverage; therefore, the risk for transmission to other low-vaccination communities and for an outbreak in the United States is heightened.^[9]

Incidence

Prevaccination cases of poliovirus reached a peak in 1952 with more than 21,000 cases of paralytic disease.^[10] In the United States, the last known case of wild virus polio infection occurred in 1979.^[10,11]

For the year 2005, the number of countries with endemic polio decreased from 7 to 4 compared with 2002 and from 125 compared with 1988.^[12] Eight countries that were polio-free were reinfected, although transmission has been reduced in all except Somalia. These 8 countries were Angola, Bangladesh, Chad, Ethiopia, Indonesia, Nepal, Somalia, and Yemen. Polio remains endemic in 3 Asian countries -- Afghanistan, India, and Pakistan. Routine vaccination coverage for infants is estimated at 80% worldwide varying from 69% in Africa to 94% in Europe. Worldwide there were 1948 virus-confirmed cases of polio in 2005, with none reported in America, Europe, or the Western Pacific.^[13]

One risk for infection is contact with low-vaccinated communities.^[9] There is a risk for vaccine-associated paralytic poliomyelitis (VAPP) seen with the live attenuated poliovirus vaccine. The risk is estimated at 1 in 2.4 million does of OPV, with first-dose risk of 1 in 750,000.^[11,14] The last case of VAPP in the United States was in 1999.^[10]

Pathogenesis

Poliovirus is an RNA picornavirus and enterovirus.^[10] A highly contagious virus, it is spread from person to person by the fecal-to-oral route by contact with stool, contaminated sewage or water, oral secretions, or fomites.^[9] After exposure, the virus replicates in the oropharynx and intestine with a resultant viremia that can infect the central nervous system. There are 3 serotypes -- 1, 2 and 3 -- with most infections caused by type 1, type 3, and type 2 in that order. The incubation period is 6-20 days (range, 3-35), and infectivity lasts 4-6 weeks. More than 90% of household contacts of an individual with wild poliovirus infection become infected. Humans are the only reservoir, and infection results in lifelong immunity.^[10,14] Long-term carriage can occur in immunodeficient individuals, but is otherwise rare.^[14] Poliovirus peaks in the summer months for temperate climates with no seasonal variation in tropical zones.^[10]

Clinical Manifestations

The majority, up to 95%, of poliovirus infections are asymptomatic. Some infections, 4% to 8%, present as a mild febrile illness that may take the form of an upper respiratory tract infection, a gastrointestinal illness, or a flulike illness. Finally, 1% to 2% present as a mild prodromal illness followed by aseptic meningitis. Less than 1% or 1 in 200 infections result in acute flaccid paralysis, with a 2- to 3-day prodromal illness followed in 1-10 days by an asymmetric paralysis.^[9-11,14] The virus has an affinity for anterior horn cells and the brainstem, resulting in spinal (79% of cases), bulbar (2%), or a combination disease (19%).^[10,14] Maximum paralysis occurs in 2-4 days with fever and muscle pain. Most patients recover muscle function. Persistent weakness or paralysis 12 months post infection is usually permanent.^[10]

The case-fatality rate for paralytic illness is 2% to 5% for children, 15% to 30% for adults, and 25% to 75% for bulbar disease.^[10] Postpolio syndrome, consisting of muscle pain and progressive or new weakness or paralysis, occurs in 25% to 40% of individuals infected in childhood and presents 30-40 years after initial infection, and is related to wild poliovirus infections.^[14]

Diagnosis

Stool cultures have the greatest yield for poliovirus. Individuals suspected of polio should have 2 stool cultures and 2 throat swabs obtained 24 hours apart early in the course of illness, along with serotyping. Acute- and convalescent-phase serum should be tested for antibody to each of the 3 serotypes.^[10,11]

Treatment

There is no antibiotic therapy for poliomyelitis and treatment is supportive and symptomatic. Bed rest prevents progression of the paralysis and heat may reduce muscle pain and spasm. Ventilatory support should be initiated as warranted. Physical therapy can begin as soon as progression in paralysis ends. Patients may need long-term physical and psychological support.^[15]

Prevention

All infants in the United States should receive inactivated poliovirus vaccine (IPV). OPV was phased out in the United States by 2000 on the basis of continued occurrence of VAPP, the lack of endemic disease, and the lowered risk for wild poliovirus importation.^[10,14] Routine vaccination is not recommended for adults, except for certain individuals who are at greater risk or are unvaccinated. IPV should be used for immunodeficient individuals.^[10] Vaccine recommendations for poliovirus vaccine include the following:^[14]

1. Routine vaccination of infants and children at 2, 4, 6-18 months, and 4-6 years;
   
    
2. Incompletely vaccinated children;
   
    
3. Travelers to areas or countries where polio is endemic or epidemic;
   
    
4. Immunocompromised individuals;
   
    
5. Communities or population groups with disease caused by wild poliovirus;
   
    
6. Laboratory workers who handle poliovirus specimens;
   
    
7. Healthcare workers who have contact with patients excreting wild poliovirus; and
   
    
8. Unvaccinated adults whose children will receive oral poliovirus vaccine.

Current vaccination rates in the United States are approximately 95%. A serotype survey of low-income children age 19-35 months, conducted from 1997 to 1998, indicated that antibody levels to serotypes 1, 2, and 3 were 96.8%, 99.8%, and 94.5%, respectively.^[11,14] Members of certain religious groups objecting to vaccination remain at risk. These include Christian Scientists and Amish communities, where outbreaks occurred in 1972 and 1979, respectively.^[11]

Two poliovirus vaccines are licensed in the United States -- enhanced inactivated poliovirus vaccine IPOL (Sanofi Pasteur, Swiftwater, Pennsylvania) and Poliovax (Sanofi Pasteur, West Toronto, Ontario, Canada), but only IPOL is manufactured and distributed in the United States.^[14] See Table 4 for poliovirus vaccine schedule.

Table 4. Recommended Vaccine Schedule for Inactivated Poliovirus Vaccine (IPV), United States

Vaccine	Recipient Age	Dose (mcg)	Volume†	Number of Doses	Schedule
IPOL* IPV	Infants <1 year to 18 mos	--	0.5 mL/SQ	3	Give at age 2, 4, 6-18 mos (may be given a minimum of 4 weeks apart)
	4-6 years	--	0.5 mL/SQ	1	May be given as early as 18 weeks
	7-18 years	--	0.5 mL/SQ	3	Give first dose, second dose 1-2 mos after first, and third dose 6-12 mos after first dose; may be given a minimum of 4 weeks apart
	> 18 years		0.5 mL/SQ	3	Give first dose, second dose 1-2 mos after first, and third dose 6-12 mos after first dose; may be given a minimum of 4 weeks apart
Pediarix† HepB/DTaP/IPV	Infants <1 year	10	0.5 mL/IM	3	Give at age 2, 4, 6 mos
	1-6 years	10	0.5 mL/IM	3	††1st dose followed by 2nd dose 1-2 mos after first (minimum 4 weeks); 3rd dose 6 mos after 1st – Use only for primary series not for booster doses

 

*IPOL (Sanofi Pasteur, Swiftwater, Pennsylvania)
^†Pediarix (GlaxoSmithKline, Research Triangle Park, North Carolina); Hep B = hepatitis B surface antigen; DTaP = acellular diphtheria pertussis
^‡SQ = subcutaneously; IM = intramuscularly; anterolateral thigh in infants 7/8-in needle, deltoid muscle with needle length appropriate for size (minimum, 1 in)
^§May be used in infants who received birth dose of hepatitis B vaccine for primary series.
mcg = micrograms
From: US Centers for Disease Control and Prevention (CDC). Poliomyelitis. In: Epidemiology and Prevention of Vaccine Preventable Diseases: The Pink Book. 10th ed. Atlanta: CDC; 2007:101-114. Available at: http://www.cdc.gov/vaccines/pubs/pinkbook/downloads/polio.pdf Accessed May 16, 2007.
From: US Centers for Disease Control and Prevention (CDC). Poliomyelitis prevention in the United States. MMWR Recomm Rep. 2000;49:1-22. Available at: http://www.cdc.gov/mmwr/preview/mmwrhtml/rr4905a1.htm Accessed May 16, 2007.
From: 2007 Childhood & Adolescent Immunization Schedules. Available at: http://www.cdc.gov/nip/recs/child-schedule.htm Accessed April 11, 2007.

Efficacy studies have indicated that 90% to 100% of children develop protective antibodies after 2 doses and 99% to 100% after the 3-dose primary series.^[14]

Vaccine should not be given to individuals with a severe allergic reaction to vaccine components, streptomycin, polymixin B, and neomycin. Serious adverse events have not been documented for enhanced IPV, which is a greater immunogenic formulation of IPV for children and adults than previous formulations of IPV. Vaccination for pregnant women is not recommended unless the woman is at increased for poliovirus infection. Vaccine may be given while breastfeeding, with minor upper respiratory infections, with a history of mild-to-moderate local reaction, with current antibiotic treatment, and during the convalescent phase of an acute illness. OPV or IPV does not increase the risk for GBS.^[10,14]

OPV is recommended for mass vaccination during polio outbreaks. In the United States, any case of paralytic poliomyelitis should be reported. No vaccination coverage is required if VAPP, but if wild poliovirus is identified then unvaccinated individuals in the community require vaccination.^[10] For individuals who have received the primary series of 3 vaccinations, a booster dose of OPV or IPV may be given.^[14]