Pan American Health Organization Working With Countries to Investigate and Control Outbreak of Polio Derived from Sabin Vaccine Virus in Dominican Republic and Haiti

Washington, December 8, 2000—The Pan American Health Organization has 16 epidemiologists supporting the Ministries of Health of the Dominican Republic and Haiti in the investigation of a current outbreak of poliomyelitis and in carrying out vaccination campaigns on the island of Hispaniola, which both countries share, according to Dr. Ciro de Quadros, who directs the Organization’s Division of Vaccines and Immunization.

The outbreak 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). The Pan American Health Organization convened a group of immunization experts to discuss these concerns raised by the outbreak.

In a report presented at the meeting of experts, PAHO officials said that because of the concern of possible pesticide poisoning in Constanza, an agricultural area of the Dominican Republic, an active case search was initiated after a case of acute flaccid paralysis was reported in early October, and two more paralysis cases were found. Lab testing showed than an unusual strain of Sabin Type 1 poliovirus was isolated, and 18 possible cases were being investigated, with 14 of them being clinically compatible with poliomyelitis. The cases are located in the provinces of La Vega, Santiago, and Mons Nouel, with 13 of them concentrated in Constanza City. Except for one case, all the others are epidemiologically linked, the report said. In Haiti, a single laboratory-confirmed case due to the derived type 1 virus has been reported to date, in the town of Nan Citron, with paralysis onset on August 30.

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. Experts from PAHO have been sent to Hispaniola to work with local authorities in the investigation, and 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.

After intensive case-finding activities, no other cases have been found so far, Dr. de Quadros said, and all of the cases were either unvaccinated or had not had their complete doses of polio vaccine. The essential 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,” he added.

The experts at the PAHO meeting, who included Dr. D.A. Henderson, leader of the global smallpox eradication campaign and chairman of PAHO's Technical Advisory Group, discussed the steps being taken to determine the causes of the virus reversion, to determine the extent of transmission, to control the outbreak, and possible implications for the global polio eradication campaign, which aims to rid the planet of the disease by 2005.

Dr. Henderson said, "We will continue studying this outbreak, but meanwhile we need to proceed ahead with the global eradication campaign. This episode, an outbreak from a virus derived from a vaccine strain that made a transition two years ago, occurred in an area where vaccination levels were low. The lesson is clear. We must keep vaccination coverage high until we get to the zero point of stopping polio transmission, we must undertake additional studies, and we must keep everyone updated on this situation."

Since the Global Polio Eradication Initiative was launched in 1988, the number of polio cases globally has dropped by over 95%, from an estimated 350,000 in 1988 to 7094 reported in 1999. There have only been some 1481 confirmed cases of polio so far this year. The WHO European Region (made up of 51 countries, including the Commonwealth of Independent States) has not had any new cases of indigenous polio for almost two years. The Western Pacific Region of WHO was certified as polio free in October, becoming the second region after the Americas, which was declared polio free in 1994.

The virus detected in the Dominican Republic was 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). It 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 99.5 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, according to CDC experts.

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 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 was founded in 1902, works to improve health and raise living standards in all the countries of the Americas. The Organization launched its drive to eradicate polio from the Americas in 1985. It also serves as the Regional Office for the Americas of the World Health Organization