Update on Vaccine-Derived Polioviruses --- Worldwide, July 2009--March 2011

Update on Vaccine-Derived Polioviruses --- Worldwide, July 2009--March 2011
Weekly
July 1, 2011 / 60(25);846-850


In 1988, the World Health Assembly resolved to eradicate poliomyelitis worldwide (1). The live, attenuated oral poliovirus vaccine (OPV) has many advantages favoring its use in polio eradication: it is administered easily by mouth; confers intestinal immunity, making recent OPV recipients resistant to infection by wild polioviruses (WPVs); provides long-term protection against paralytic disease through durable humoral immunity; and is inexpensive. Despite its many advantages, OPV use carries the risk for occurrence of rare cases of vaccine-associated paralytic poliomyelitis among immunologically normal OPV recipients and their contacts and the additional risk for emergence of vaccine-derived polioviruses (VDPVs). Because of these risks, OPV use will be discontinued worldwide once the goal of eradicating all WPV transmission is achieved. VDPVs can cause polio outbreaks in areas with low OPV coverage and can replicate for years in immunodeficient persons; therefore, strategies to strengthen global polio immunization and surveillance are needed to limit emergence of VDPVs (2). This report updates previous surveillance summaries (3,4) and describes VDPVs detected worldwide during July 2009--March 2011 and reported as of June 20, 2011. Three new outbreaks of circulating VDPVs (cVDPVs), ranging in size from six to 16 cases, were identified in Afghanistan, Ethiopia, and India; three previously identified outbreaks in Nigeria, Democratic Republic of Congo (DRC), and Somalia continued through late 2010 or into 2011 and resulted in 355, 37, and 13 total cases, respectively; two countries experienced importations of cVDPVs from Nigeria; nine newly identified paralyzed immunodeficient persons in seven middle-income and developing countries were found to excrete VDPVs; and VDPVs were found among persons and environmental samples in 15 countries. With the use of alternate OPV formulations since 2005 (1) and with enhanced poliovirus surveillance sensitivity and laboratory screening, the number of identified cVDPV outbreaks per year has increased over time (2,3). To prevent VDPV emergence and spread, all countries should maintain high poliovirus vaccination coverage against all three poliovirus serotypes. Sensitive poliovirus surveillance to detect VDPVs will continue to increase in importance.

Properties of VDPVs

VDPVs can cause paralytic polio in humans and have the potential for sustained circulation. VDPVs resemble WPVs biologically (3) and differ from the majority of vaccine-related poliovirus (VRPV) isolates by having genetic properties consistent with prolonged replication or transmission. Because poliovirus genomes evolve at a rate of approximately 1% per year, VRPVs that differ from the corresponding OPV strain by >1% of nucleotide positions (usually determined by sequencing the genomic region that encodes the major viral surface protein [VP1]) are presumed to have replicated for at least 1 year in one or more persons after administration of an OPV dose. This is substantially longer than the normal period of vaccine virus replication of 4--6 weeks in an OPV recipient.

Three poliovirus serotypes exist: types 1, 2, and 3. Poliovirus isolates are grouped into three categories, based on the extent of divergence of the VP1 nucleotide region compared with the corresponding OPV strain: 1) VRPVs (<1% divergent [types 1 and 3] or <0.6% divergent [type 2]); 2) VDPVs (VRPVs that are >1% divergent [types 1 and 3] or >0.6% divergent [type 2] from the corresponding OPV strain); and 3) WPVs (no genetic evidence of derivation from any vaccine strain) (3). VDPVs are further categorized as 1) cVDPVs, when evidence of person-to-person transmission in the community exists; 2) immunodeficiency-associated VDPVs (iVDPVs), which are isolated from persons with primary immunodeficiencies who have prolonged VDPV infections; and 3) ambiguous VDPVs (aVDPVs), which are either clinical isolates from persons with no known immunodeficiency or sewage isolates whose source is unknown (3).

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Update on Vaccine-Derived Polioviruses --- Worldwide, July 2009--March 2011