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Back To Vidyya Outbreak Of Poliomyelitis In Dominican Republic And Haiti:

Low Vaccination Coverage With Oral Polio Vaccine Allows Sabin-Derived Vaccine To Circulate

A current outbreak of poliomyelitis in Dominican Republic and Haiti has raised serious concerns because the Western Hemisphere has been free of wild poliovirus circulation since 1991, and because the virus identified is an unusual derivative of the Sabin type 1 oral poliovirus vaccine (OPV), according to Dr. Ciro de Quadros, who directs the Pan American Health Organization's Division of Vaccines and Immunization.

The Ministries of Health of the Dominican Republic and Haiti, with the assistance of the Pan American Health Organization (PAHO) and the Centers for Disease Control and Prevention (CDC), are investigating the outbreak to determine the extent of spread and evaluate the reasons for the outbreak. Aggressive control measures have already been put in place. A mass vaccination campaign with OPV has already started in the Dominican Republic, initially covering the three provinces with suspected cases, followed shortly by the rest of the country. In Haiti, three nationwide vaccination rounds with OPV are planned for January, February and March.

Since July 12, 2000, a total of 3 laboratory-confirmed cases due to derived poliovirus type 1 isolates have been identified, Dr. de Quadros said. An additional 16 persons with acute flaccid paralysis (AFP) are now under investigation in the Dominican Republic. In Haiti, a single laboratory-confirmed case due to the derived type 1 virus has been reported to date, with paralysis onset on August 30. After intensive case-finding activities, no other cases have been found so far. The virus detected, first isolated by the PAHO Poliovirus Laboratory at the Caribbean Epidemiology Center and subsequently characterized at the Poliovirus Laboratory at the Centers for Disease Control and Prevention (CDC) is unusual because it is derived from OPV, has 97 percent genetic similarity to the parental OPV strain (normally OPV derived viruses are greater than 9.5 percent), and appears to have assumed the characteristics of wild poliovirus type 1, both in terms of neurovirulence and transmissibility. The difference in nucleotide sequence suggests the virus has been either replicating for a prolonged period in an immunodeficient individual, or circulating for as long as two years in an area where vaccination coverage is very low, resulting in ongoing genetic changes in the original Sabin virus that gave it the properties of wild poliovirus.

Prolonged circulation of OPV-derived polioviruses in areas with very low OPV coverage has been documented in only one other setting--type 2 OPV-derived virus circulated in Egypt for an estimated 10 years (1983-1993) and was associated with more than 30 reported cases. In this instance, vaccination coverage was very low in the affected areas, and circulation of a vaccine-derived poliovirus was terminated rapidly once OPV vaccination coverage increased.

The key factor for control of circulating OPV-derived viruses is the same as that required to control wild poliovirus circulation: achieving and maintaining high vaccination coverage, Dr. de Quadros said No evidence for circulation of OPV-derived virus has ever been found in any area with high coverage.

"The current outbreak is a powerful reminder that even polio-free areas need to maintain high coverage with polio vaccine until polio eradication has been achieved," Dr. de Quadros said.

"Nearly four decades of experience with oral polio vaccine has shown that it is very safe and effective in preventing poliomyelitis. OPV is the vaccine of choice for the eradication of wild polioviruses. However, it is crucial to maintain high OPV coverage to protect against imported wild polioviruses and to prevent person-to-person transmission of OPV-derived viruses," he added.

"It is also important that all countries maintain high quality AFP and poliovirus surveillance, that current activities to complete the global eradication of wild polioviruses be accelerated, and that a global strategy is developed for the orderly cessation of immunization with OPV after global certification of polio eradication is achieved,"Dr. de Quadros said. 

Travelers to the Dominican Republic and Haiti who are not adequately immunized must be considered at risk of acquiring poliomyelitis, and should make certain they are fully immunized against polio. Those countries using OPV for routine immunization recommend at least a 3-dose primary vaccination series.

PAHO, which also serves as the Regional Office for the Americas of the World Health Organization, works to improve health and raise living standards in all the countries of the Americas.

For further information, contact: Daniel Epstein, tel (202) 974-3459 , or (301) 897-0734, fax (202) 974-3143, Pan American Health Organization, Office of Public Information, email epsteind@paho.org, Internet: www.paho.org


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Editor: Susan K. Boyer, RN
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