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Vaccine-derived polioviruses
(VDPV) are defined as live, attenuated strains of the virus contained in
the oral polio vaccine (OPV) which have changed and reverted to a
form that can cause paralysis in humans with the capacity for sustained
circulation. VDPVs
differ from the parental (original) Sabin strains found in the
vaccine by 1 to 15% of VP1
nucleotides. This is a measurement of genetic change that
scientists use to monitor the circulation of viruses.
-
cVDPVs
(circulating vaccine-derived
polioviruses) that are associated with sustained person-to-person transmission
and considered to be circulating in the environment;
-
iVDPVs
(immunodeficiency related vaccine-derived poliovirus) isolated from immunodeficient patients who have prolonged
infections after exposure to OPV; and
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aVDPVs
(ambiguous vaccine-derived poliovirus) isolated from a single
immunocompetent AFP or paralytic poliomyelitis patient with or without
additional isolates from contacts, or from healthy individuals or the
environment in absence of paralytic cases.
Management
of VDPVs
On
very rare occasions, in areas where polio immunization coverage
has been low, vaccine viruses have regained the ability to
circulate in a population, and cause paralysis.
Between
1997 and 2007, over 10 billion doses of OPV were administered worldwide. In the same period,
nine cVDPV outbreaks in
nine countries were confirmed, resulting in
fewer than 200 polio cases (table
in PDF). By
contrast, during this same period, wild poliovirus has caused more
than 33,100 cases worldwide.
Experience
shows that cVDPVs outbreaks can usually be interrupted rapidly
through the implementation of high-quality
supplementary immunization activities (SIAs) using OPV.
Immunodeficiency
related vaccine-derived poliovirus (iVDPVs)
Prolonged
intestinal replication of OPV viruses has also been observed in a
limited number of persons with rare immune deficiency disorders.
Some of these patients have not been able to clear the intestinal
vaccine virus infection, and have excreted
immune-deficiency-related vaccine-derived polioviruses (iVDPVs)
for prolonged periods.
The
occurrence of iVDPVs also appears to be very rare, with only 33
patients with the condition documented around the world. Of these,
most have stopped excretion after a period or have died. Only
three are known to have excreted for >5years; with two of these
continuing to excrete virus when last examined.
More
importantly, no iVDPV has been observed to transmit or spread to others. For
this reason, the public health significance of 'chronic iVDPV excretors' is
different when compared to circulating VDPVs. However, iVDPVs may pose an
increasing risk as the world moves towards the eventual cessation of OPV use.
Risk
Factors for VDPV Emergence
The overriding factor for the
emergence of all VDPVs is the same as for wild poliovirus (WPV) circulation, low
immunity levels in a population. Where routine coverage with OPV is poor or
where there is an absence of high quality supplementary immunization activities,
a population can become susceptible to the emergence of VDPV. This is because
the virus has time to change, replicate and exchange genetic material with other
enteroviruses, while spreading through a population.
Importantly, if a population is
fully immunized against polio, it will be protected against the spread of both
wild and vaccine strains of poliovirus.
Implications and management of VDPVs
VDPVs
do not appear to be as robust or transmissible as wild poliovirus.
Studying each outbreak has shown that most were either
self-limiting or rapidly stopped with OPV campaigns.
Wild polioviruses pose a much greater danger to populations
than VDPVs. As long as
wild poliovirus circulates anywhere in the world, it is vital to
maintain high population immunity levels.
Maintaining high population immunity will also minimize the
risk of cVDPVs occurring.
The
management of cVDPVs is a necessary part of the global polio
eradication effort, and is similar to management of wild
poliovirus outbreaks; i.e. by rapid implementation of large-scale,
high-quality SIAs.
OPV
is the polio vaccine by which to achieve the eradication of wild
polioviruses worldwide.
Once
wild poliovirus transmission has been interrupted globally,
however, OPV use in routine immunization programmes will
eventually be stopped, to eliminate also the rare
risks posed by VDPVs and as recommended by the Advisory Committee
on Polio Eradication.
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