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Polio Disease Transmission Model

Revision as of 18:00, 23 March 2012 by (Talk | contribs)

Polioviruses Transmission Model [1]

Global efforts to eradicate wild polioviruses continue, with types 1 and 3 wild polioviruses remaining endemic in three countries (Nigeria, Afghanistan, Pakistan). Even though India has recently (since 13 January 2011, no case of wild poliovirus and detected the virus in sewage sampling,) been considered to have interrupted transmission of indigenous wild poliovirus [2]. Critical need to maintain immunity to poliovirus in India until global eradication achieved.

Wild polioviruses still cause fewer than 2000 global cases of paralytic polio annually [3]. While wild polioviruses circulate in these areas, the rest of the world must continue to keep polio vaccination levels very high [4], due to the risk of outbreaks in susceptible people in polio-free countries. In addition, post-eradication policy planning must anticipate that outbreaks (defined as one or more cases of paralytic polio) will occur after the successful disruption of wild poliovirus transmission [5,6], largely due to the risks of circulating vaccine-derived polioviruses (cVDPVs) [7] (mainly because of the utilization of oral polio vaccine, OPV). Most people infected with poliovirus do not show any symptoms, which necessitates modeling the transmission of infections [7], but about 1/200 susceptible people becomes paralyzed from a wild poliovirus infection [8-10]. The costs of outbreaks include both health costs experienced by paralyzed individuals plus the impacts on their families, and the financial costs associated with treating patients and responding to the outbreak with vaccine campaigns to reduce transmission [11-13].

Two vaccines provide protection from disease (paralytic poliomyelitis), but incomplete protection from infection: live oral poliovirus vaccine (OPV) and inactivated poliovirus vaccine (IPV). OPV represents the vaccine of choice for the Global Polio Eradication Initiative because of its low cost, ease of administration, induction of mucosal immunity, and ability to provide secondary protection (i.e. spread to contacts), so called contact vaccination. However, OPV can cause vaccine associated paralytic polio (VAPP) in rare cases and lead to outbreaks with cVDPVs in populations with large numbers of susceptibles, and consequently following the successful eradication of wild polioviruses global health leaders plan to eliminate the use of OPV [14]. Minimizing the risks of outbreaks will require coordination of OPV cessation, creation of a global vaccine stockpile, and development of specific plans for outbreak response [15,16]. Many countries will also consider switching from OPV to IPV because it carries no risk of vaccine-associated polio paralysis, but IPV represents a relatively expensive choice and its ability to prevent poliovirus transmission in some settings (notably low-income areas with relatively poor hygiene and inadequate health systems) remains uncertain [5, 6].

The polio model in STEM captures both wild polio transmission and the risks of circulating vaccine-derived polioviruses (cVDPVs) from OPV (OPV reversion) using nine compartments (Fig. 1). Back mutation or reversion is the process that the live virus upon replicating in its human host, may regain its virulence and transmissibility, potentially causing infection in the vaccine and hosts’ contacts.(Fig. 1).


Fig. 1 Compartment Model of Polio Transmission in STEM


  • So - fully susceptible
  • Sv - partically susceptible, means historically has been exposed to vaccine derived poliovirus
  • Ew - exposed state after contracting wild polio virus
  • Ev - exposed state after getting OPV derived poplio virus
  • Iw - infectious state, is able to shedding wild poliovirus
  • V - vaccinated state
  • R - recovery state either by getting vaccine or recovering from wild poliovirus infection
  • Pw - wild poliovirus derived paralytic polio cases
  • Pv - vaccine (OPV) derived paralytic polio cases (for VAPP)




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