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
"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 firstname.lastname@example.org,