Progress Toward Interruption of Wild Poliovirus Transmission --- Worldwide, 2009

Progress Toward Interruption of Wild Poliovirus Transmission --- Worldwide, 2009
Weekly
May 14, 2010 / 59(18);545-550


In 1988, an estimated 350,000 cases of poliomyelitis were occurring annually worldwide. By 2005, because of global vaccination efforts, indigenous transmission of wild poliovirus (WPV) types 1 and 3 (WPV1 and WPV3) had been eliminated from all but four countries (Afghanistan, India, Nigeria, and Pakistan). No cases of WPV type 2 have been reported since 1999. This report describes progress toward global WPV eradication during 2009 and updates previous reports (1--6). During 2009 a total of 1,606 cases of WPV infection were reported, compared with 1,651 in 2008. WPV3 incidence increased 67%, to 1,124 cases, compared with 675 in 2008. However, WPV1 incidence decreased 51%, to 482 cases in 2009, compared with 976 cases in 2008. In India, nearly all polio cases in 2009 were reported in high-risk districts in western Uttar Pradesh and central Bihar. In Afghanistan and Pakistan, WPV circulation in high-risk districts continued because of difficulties vaccinating children in conflict-affected areas and operational limitations in parts of Pakistan (5). In Nigeria, cases decreased by 51%, to 388 cases in 2009, compared with 798 in 2008. During 2009, outbreaks from importation of WPV affected 19 previously polio-free African countries (2). Two key steps are needed to make further progress in polio eradication: 1) addressing local barriers to interrupting transmission, and 2) using bivalent oral poliovirus vaccine (bOPV) broadly for WPV 1 and 3 in supplemental immunization activities (SIAs).

Routine Vaccination

Global routine vaccination coverage of infants with 3 doses of trivalent oral poliovirus vaccine (tOPV) by age 12 months was estimated at 83% in 2008,* and coverage varied by World Health Organization (WHO) region: African (72%), South-East Asian (73%), Eastern Mediterranean (84%), Americas (92%), European (96%), and Western Pacific (97%). Estimated national 3-dose tOPV coverage for 2008 was 85% in Afghanistan, 81% in Pakistan, 67% in India, and 61% in Nigeria. However, routine 3-dose tOPV coverage of <40% was reported from the Indian states of Bihar and Uttar Pradesh, parts of Afghanistan and Pakistan, and the northern Nigerian states.†

Supplementary Immunization Activities

In 2009, a total of 270 oral polio vaccine (OPV) SIAs§ were conducted in 40 countries (101 national immunization days, 120 subnational immunization days, 21 child health days, and 28 mop-up rounds). An estimated 2.21 billion OPV doses were administered to approximately 360 million children aged <5 years. Of those doses, 39% were tOPV, 51% were monovalent OPV type 1 (mOPV1), 10% were monovalent OPV type 3, and <1% were bOPV. Of the 270 SIAs, 85 (32%) were conducted in the four polio-endemic countries (34 in India, 23 in Pakistan, 13 in Afghanistan, and 15 in Nigeria), 136 (50%) in countries where WPV was reintroduced in 2009 (15) or earlier (five), and 49 (18%) in 16 countries without confirmed WPV cases in 2009.

Acute Flaccid Paralysis Surveillance

The acute flaccid paralysis (AFP) surveillance system is fundamental to monitoring progress toward polio eradication. The system tracks all AFP cases in children aged <15 years and all paralytic illness cases in persons of any age when polio is suspected. The quality of AFP surveillance is monitored by WHO performance indicators.¶ In 2009, each WHO region (except for the European Region) maintained the overall sensitivity of AFP surveillance at certification-standard levels (Table). Since 2005, an operational target for all countries reporting WPV and for neighboring countries has been to achieve a nonpolio AFP rate of >2 cases per 100,000 children aged <15 years. In 2009, all four polio-endemic countries and the 19 other countries with WPV circulation reached this target nationally, although subnational surveillance quality varied substantially.

Wild Poliovirus Incidence

Of 1,606 WPV cases with onset of paralysis reported worldwide during 2009 (Table, Figure), 1,256 (78%) were from the four polio-endemic countries, 207(13%) were from 15 previously polio-free countries after WPV importation, and 143 (9%) were from four countries with reestablished transmission (transmission for >12 months after importation). WPV1 cases decreased from 976 in 2008 to 482 in 2009, whereas WPV3 cases increased from 675 in 2008 to 1,124 in 2009. The number of polio-affected districts decreased 3%, from 496 in 2008 to 481 in 2009.

India. India reported 741 WPV cases in 2009 (79 WPV1, 661 WPV3, and one mixed WPV1/WPV3), an increase compared with 559 cases in 2008. WPV transmission mainly occurred in the northern states of Uttar Pradesh (33 WPV1, 568 WPV3, and one mixed WPV1/WPV3) and Bihar (38 WPV1 and 79 WPV3). The remaining cases in six states and Delhi (eight WPV1 and 14 WPV3) resulted from importation from these two states. Environmental sampling in Mumbai detected one WPV1-positive sample in January 2009 and one WPV3-positive sample in December 2009, whereas sampling in 2008 detected two WPV1-positive samples and 31 WPV3-positive samples. All positive samples in 2008--2009 were of Bihar origin.

Afghanistan and Pakistan. Afghanistan reported 38 WPV cases in 2009 (15 WPV1 and 23 WPV3), compared with 31 WPV cases in 2008, and Pakistan reported 89 WPV cases (60 WPV1, 28 WPV3, and one mixed WPV1/WPV3), compared with 117 cases in 2008. WPV transmission was restricted primarily to previously affected districts in both countries (5). In Afghanistan, 34 (90%) WPV cases occurred in 12 high-risk districts in the conflict-affected southern region. Pakistan experienced continued WPV transmission in security-compromised areas of the Northwest Frontier Province, and in accessible areas of Balochistan and Sindh provinces, where managerial and operational limitations continued to affect vaccination coverage. During 2009, both countries continued to conduct coordinated SIAs and used multiple strategies to reach previously unvaccinated children.

Nigeria. Reported WPV cases in Nigeria decreased from 798 in 2008 (721 WPV1, 76 WPV3, and one mixed WPV1/WPV3) to 388 in 2009 (75 WPV1 and 313 WPV3). After increased involvement of state and local authorities and traditional leaders in 2008--2009, community acceptance and indicators of SIA quality improved in some previously high-incidence states in northern Nigeria. In addition, a sustained decrease in the weekly incidence of cases (particularly WPV1) occurred in the second half of 2009, especially in the northern states (4). However, surveillance monitoring for 2009 indicated that among children aged 6--35 months, up to 50% received <3 doses OPV and up to 20% received no doses in previously high-incidence northern states.

Importations. In 2009, as a consequence of importations that occurred in 2008 or earlier, WPV transmission was confirmed to be reestablished in Angola and Chad and suspected to be reestablished, based on virologic data, in the Democratic Republic of the Congo (DRC) and southern Sudan (2). During August 2008--December 2009, WPV endemic to Nigeria was exported, mostly through intermediate countries, to 10 countries in west Africa and two countries in central Africa and resulted in 178 cases in 2009.** In 2009, WPV3 transmission occurred in the Central Africa Republic through importations from Chad (transmission since 2007, originating from Nigeria) and from DRC (after transmission in Angola in 2008, originating from India) (2). WPV1 outbreaks in Kenya and Uganda in 2009 resulted from importations from southern Sudan (genetic linkage to WPV1 isolated during the outbreak in Sudan during 2004--2005, originating from Nigeria). In Burundi, two WPV1 cases were detected with genetic linkage to WPV1 isolated in DRC in 2008 (after transmission in Angola in 2008, originating from India).

Vaccine-Derived Polioviruses

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Progress Toward Interruption of Wild Poliovirus Transmission --- Worldwide, 2009