January 2001
SANTO DOMINGO, Dominican Republic - A vaccine strain of oral poliovirus vaccine (OPV) was identified as the cause of a poliomyelitis outbreak in the Dominican Republic and Haiti.
In a four-month period, six laboratory-confirmed cases of poliovirus type 1 were identified in the Dominican Republic. Out of 19 total reported cases of acute flaccid paralysis (AFP), 84% occurred in children younger than 6 years, with case-patient ages ranging from 9 months to 21 years. All were unvaccinated or inadequately vaccinated. One laboratory-confirmed case was identified in Haiti in an inadequately vaccinated 2-year-old.
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---A boy
receiving oral poliovirus vaccine in the Dominican
Republic.
The first case of AFP was reported in early October
2000. Subsequently, most cases investigated showed they were clinically
compatible with poliomyelitis. The Dominican cases were located in the
provinces of La Vega, Santiago and Mons Nouel, with most occurring in Constanza
City, according to Pan American Health Organization (PAHO) reports. All cases
were epidemiologically linked, except for one. The Haitian case occurred in Nan
Citron with paralysis onset in August 2000.
Officials at PAHO said the outbreak raises serious concerns because the Western Hemisphere has been free of wild poliovirus circulation since 1991. The ministries of health of the Dominican Republic and Haiti, the PAHO and the Centers for Disease Control and Prevention (CDC) are investigating.
"We have 16 epidemiologists looking actively to see if they can identify other cases," said Ciro de Quadros, MD, MPH, director, division of vaccines and immunization, PAHO.
"They are going into communities, schools, clinics and hospitals to investigate if there are any missing cases health care workers did not (identify). They're also helping in the organization and supervision of a mass vaccination campaign."
De Quadros said two situations could have sparked the outbreak. One is that an immunocompromised individual could have excreted the virus; but surveillance thus far does not support the theory.
The other more likely event is the outbreak began through the passage of the virus transmitted from one unimmunized person to another, and the virus reacquired some of its characteristics, virulent and transmissible, said de Quadros. He further noted that OPV coverage in the areas where outbreak cases occurred is less than 20%.
PAHO reports confirm de Quadros' hypotheses, noting the difference in the outbreak virus's nucleotide sequence suggests it's been replicating for an extended period in someone who's immunodeficient; or the virus has been circulating for as long as two years in an area where vaccination coverage is low, resulting in ongoing changes to the original OPV.
In a similar case, prolonged circulation of a type 2 OPV-derived virus occurred in Egypt for approximately 10 years. That virus was associated with approximately 30 cases, according to the PAHO.
A nationwide, three-day OPV vaccination campaign was initiated in the Dominican Republic in December, and three nationwide vaccination rounds are planned for January, February and March in Haiti.
The CDC report, which appeared in the Morbidity and Mortality Weekly Report, warned that travelers to either country who are not vaccinated adequately should be considered at risk for poliomyelitis and should be fully vaccinated according to national policies.
"It's not the major concern, but we issued an advisory to make sure that whenever people travel anywhere, not just the Dominican Republic, because there are about 30 endemic countries, they should be immunized.
"It's the same for measles. In the region of the Americas, the only countries with endemic transmission for measles are the Dominican Republic and Haiti. We recommend that people who go there be immunized."
For more information:
- CDC. Public health dispatch: Outbreak of poliomyelitis — Dominican Republic and Haiti, 2000. MMWR 2000;49(48):1094-1103.
- Naguib T, Yang SJ, Pallansch M, et al. Prolonged circulation of Sabin 2-derived polioviruses. In: Program and abstracts of progress in polio eradication: vaccination strategies for the end game. Geneva, Switzerland: International Association for Biologicals, 2000.
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