Apparent Global Interruption of Wild Poliovirus Type 2 Transmission

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In 1988, the World Health Assembly of the World Health Organization (WHO) resolved to eradicate poliomyelitis by 2000. Since then, the WHO Region of the Americas and Western Pacific Region have been certified free of polio, and the European Region is approaching 3 years since the last confirmed case of polio. Transmission of wild polio-virus types 1 and 3 continues to decline in the other WHO regions (1). This report summarizes the evidence, obtained through surveillance for acute flaccid paralysis (AFP), supporting the global interruption of wild poliovirus type 2 transmission.

Along with achieving and maintaining high routine coverage with oral poliovirus vaccine (OPV), conducting National Immunization Days* to decrease poliovirus circulation, and mopping-up vaccination activities^† to eliminate remaining reservoirs^§ of poliovirus transmission, one of the main polio eradication strategies is AFP surveillance. The quality of AFP surveillance is assessed primarily by the nonpolio AFP rate (target: >1 per 100,000 population aged <15 years), and by the completeness of specimen collection (target: two adequate stool specimens^¶ from >80% of persons with AFP).

The last countries to report wild poliovirus type 2 isolates were Afghanistan and Pakistan in 1997, Nigeria in 1998, and India in 1999 (2). The last known reservoirs of wild poliovirus type 2 transmission occurred in Bihar, Uttar Pradesh, and West Bengal in northern India. Several type 2 isolates were obtained from this region during 1998--1999. The rapidly declining genetic diversity of the few sustaining type 2 isolate chains is consistent with the final phase of transmission. The last wild poliovirus type 2 isolated was from a child reported as an AFP case in West Bihar with paralysis onset in October 1999.

Despite substantially improved AFP surveillance globally since late 1999, no wild poliovirus type 2 isolates have been reported by any WHO region since late 1999. From 1999 to 2000, the number of AFP cases reported worldwide increased from 29,583 to 30,436 despite a decrease of confirmed polio cases from 7141 in 1999 to 2824 in 2000. In the South-East Asia Region during 1999--2000, the overall nonpolio AFP rate increased from 1.6 to 1.7 per 100,000 population aged <15 years, and the rate of adequate stool collection increased from 71% to 81%, respectively. In the Eastern Mediterranean Region, the overall nonpolio AFP rate increased from 1.1 to 1.4 and the rate of adequate stool collection remained at 67%. In the African Region during 1999--2000, the overall nonpolio AFP rate increased from 0.8 to 1.3; however, the rate of adequate stool collection (53%) remained below the 2000 target level. Surveillance remains suboptimal in the major reservoir countries of Angola, Democratic Republic of Congo, Ethiopia, and Nigeria.

AFP surveillance comprises a global network of seven specialized, 15 reference, and 126 national WHO-accredited laboratories. The network processed 48,370 stool specimens in 1999 and approximately 50,000 in 2000. During 1999--2000, 1423 isolates were wild poliovirus type 1 (989 in 1999 and 434 in 2000); 11 were wild poliovirus type 2 (11 in 1999 [from India] and zero in 2000); 1127 were wild poliovirus type 3 (894 in 1999 and 233 in 2000), and 23 were wild poliovirus types 1 and 3 mixed isolates (16 in 1999 and seven in 2000) (Table 1).

Reported by: Vaccines and Biologicals Dept, World Health Organization, Geneva, Switzerland. Respiratory and Enteric Viruses Br, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases; Vaccine Preventable Disease Eradication Div, National Immunization Program, CDC.

Editorial Note:

The apparent elimination of wild poliovirus type 2 represents a milestone for the global polio eradication initiative and an indication that the current strategies can eradicate poliovirus types 1 and 3. Since late 1999, the global polio laboratory network has processed tens of thousands of stool specimens, including those from countries at high risk for undetected poliovirus circulation. All polioviruses type 2 isolated since October 1999 have been vaccine derived, and the declining genetic diversity of the last wild isolates from India is consistent with the final phase of transmission.

Before the advent of the polio vaccine, wild poliovirus type 2 had worldwide distribution. As the vaccine was introduced, particularly in temperate climates, wild poliovirus type 2 transmission disappeared quickly. Transmission continued in countries with high population density and poor sanitation, but disappeared more quickly than other polio-virus types as vaccination rates improved. The high immunogenicity of type 2 polio-viruses in OPV and the efficient spread of the vaccine-derived strain from vaccinated persons to close contacts may be important factors in its earlier disappearance.

Although the likelihood of undetected transmission decreases with time, evidence of interruption of type 2 transmission is reinforced with continued improvement in AFP surveillance, particularly in Africa, where the nonpolio AFP rate and rate of timely specimen collection remain inadequate in some high-risk countries. In addition, the increased laboratory workload generated by improving stool collection rates must be met with additional human and financial resources to maintain the quality and timeliness of specimen processing.

Although wild polioviruses types 1 and 3 have been more difficult to control than type 2, the experience in the Americas, Western Pacific, and Europe underscores the feasibility of global eradication of all wild poliovirus serotypes.

References

1. CDC. Progress toward global poliomyelitis eradication, 1999. MMWR 2000;49:349--54.
2. CDC. Progress toward the global interruption of wild poliovirus type 2 transmission, 1999. MMWR 1999;48:736--9.

* Nationwide mass campaigns during a short period (days to weeks), in which two doses of OPV are administered to all children in the target age group (usually aged <5 years), regardless of vaccination history, with an interval of 4--6 weeks between doses.

^† Focal mass campaigns in high-risk areas during a short period (days to weeks) in which two OPV doses are administered to all children in the target age groups, regardless of vaccination history, with an interval of 4--6 weeks between doses.

^§ Countries where polio is endemic that have large populations and that may export poliovirus to neighboring countries and elsewhere.

Table 1


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