What is the epidemiology of poliomyelitis (polio) and how is it addressed?

Epidemiology of Poliomyelitis and Its Control Strategies

Poliomyelitis is a highly infectious viral disease that has been nearly eradicated globally through comprehensive vaccination programs, with remaining endemic transmission limited to specific regions in South Asia and Sub-Saharan Africa. 1

Disease Characteristics and Transmission

• Poliomyelitis is caused by three serotypes of poliovirus (types 1,2, and 3), with type 1 most frequently causing paralytic disease, followed by type 3, and least frequently type 2 1
• The virus is primarily transmitted through the fecal-oral route, but can also spread via infectious saliva, contaminated sewage, or water 1, 2
• Incubation period typically ranges from 7-21 days (range: 4-30 days) before paralytic manifestations appear 1
• Communicability begins with viral replication and excretion in oral secretions and feces, continuing for approximately 4-6 weeks after infection 1

Clinical Manifestations

• 90-95% of poliovirus infections are asymptomatic 2
• 4-8% of infections result in nonspecific illness with low-grade fever and sore throat (minor illness) 2
• 1-5% develop aseptic meningitis a few days after minor illness resolves 2
• 0.1-2% experience rapid onset of asymmetric acute flaccid paralysis 2
• Approximately 1 per 1,000 infections result in residual paralytic disease involving motor neurons 2, 3

Historical Trends and Vaccination Impact

• Before poliovirus vaccines were introduced in 1955, large outbreaks occurred annually in the United States 2
• The annual incidence of paralytic disease declined from 11.4 cases/100,000 population to 0.5 cases/100,000 after inactivated poliovirus vaccine (IPV) introduction in 1955 2
• Further decline to 0.002-0.005 cases/100,000 occurred after oral poliovirus vaccine (OPV) introduction in 1961 2
• In the United States, cases declined from >20,000 in 1952 to <100 in the mid-1960s 1
• Between 1980-1994,127 cases of paralytic poliomyelitis were reported in the US: 6 imported cases, 2 indeterminate cases, and 119 vaccine-associated paralytic poliomyelitis (VAPP) cases 2, 4

Global Eradication Efforts

• In 1988, the World Health Assembly resolved to eradicate polio globally by 2000 1
• This followed the regional goal to eliminate polio by 1990 set in 1985 by countries of the Western Hemisphere 1
• The last case of wild poliovirus in the Americas was reported in Peru in 1991, with the entire Western Hemisphere certified polio-free in 1994 1, 5

Global Eradication Strategies

The following strategies have been implemented worldwide in polio-endemic countries 1:

1. High vaccination coverage: Achieving and maintaining high coverage with at least three doses of OPV among infants under 1 year
2. Surveillance systems: Developing sensitive epidemiologic and laboratory surveillance, including acute flaccid paralysis (AFP) surveillance
3. National Immunization Days: Administering supplemental OPV doses to children under 5 years to rapidly decrease poliovirus circulation
4. Mopping-up campaigns: Conducting localized home-to-home vaccination campaigns in high-risk areas to eliminate remaining chains of transmission

Global Progress

• Global coverage with at least three doses of OPV among infants under 1 year reached 80% by 1998 1
• By 1998,90 countries conducted either National or Sub-National Immunization Days, providing supplemental OPV to approximately 470 million children under 5 years 1
• Reported polio cases globally decreased from 35,251 in 1988 to 6,227 in 1998, an 82% reduction 1, 5
• The number of polio-endemic countries decreased from >120 in 1988 to approximately 50 in 1998 1
• By 1999, approximately 50% of the world's population resided in polio-free areas, including the Western Hemisphere, Western Pacific Region (including China), and European Region 1, 5

Remaining Endemic Areas and Priority Countries

• As of 1998-1999, two large endemic areas of continued poliovirus transmission existed in South Asia and Sub-Saharan Africa 1
• Priority countries targeted for accelerated implementation included 1:
  • Seven reservoir countries: Bangladesh, Democratic Republic of the Congo, Ethiopia, India, Nepal, Nigeria, and Pakistan
  • Eight countries in conflict: Afghanistan, Angola, Democratic Republic of the Congo, Liberia, Sierra Leone, Somalia, Sudan, and Tajikistan

Vaccine-Associated Paralytic Poliomyelitis (VAPP)

• VAPP emerged as a rare but serious complication of OPV use 1
• Before sequential IPV-OPV schedule was introduced, 132 VAPP cases were reported in the US during 1980-1995 1
• The overall risk for VAPP is approximately one case per 2.4 million OPV doses distributed, with first-dose risk of one case per 750,000 first doses 1, 4
• Among immunocompetent persons, 83% of VAPP cases in vaccine recipients and 63% in contacts occurred after the first dose 1
• The US adopted an all-IPV schedule in 1999 to eliminate VAPP cases 2, 6

Current Status and Challenges

• Wild type 2 poliovirus has not caused a case since 1999 and type 3 since 2012 6
• Most infections are asymptomatic, requiring sophisticated environmental surveillance to ensure virus eradication 6
• The live vaccine (OPV) can sometimes revert to virulent circulating forms under conditions not fully understood 6
• Cessation of vaccination is an increasingly important issue, with inactivated polio vaccine (IPV) playing a larger role in the end game 6

Prevention in Non-Endemic Countries

• Universal vaccination of infants and children is essential to establish and maintain population immunity against polio 1
• Importation of wild poliovirus remains a risk until global eradication is achieved, as demonstrated by cases in Canada in 1993 and 1996 linked to virus from the Netherlands and India 1

The global polio eradication initiative represents one of the most successful public health campaigns in history, with the disease nearly eliminated worldwide through comprehensive vaccination strategies and surveillance systems. 5, 3