THE PROBABILITY OF UNDETECTED WPV CIRCULATION AFTER APPARENT GLOBAL INTERRUPTION OF TRANSMISSION

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THE PROBABILITY OF UNDETECTED WPV CIRCULATION AFTER APPARENT GLOBAL INTERRUPTION OF TRANSMISSION

The Probability of Undetected Wild Poliovirus Circulation After Apparent Global Interruption of Transmission
by Dominika A. Kalkowska, Radboud J. Duintjer Tebbens, and Kimberly M. Thompson, American Journal of Epidemiology 2012; doi:10.1093/aje/kwr399. PDF

What are the study’s main findings?

As the Global Polio Eradication Initiative drives the number of paralytic poliomyelitis cases caused by wild polioviruses (WPVs) toward zero, questions arise about the possibility of undetected or silent circulation of infection after apparent eradication, particularly since most poliovirus infections occur asymptomatically.
Concerns about silent circulation of WPVs depend on first achieving the goal of apparent WPV extinction, and our analysis demonstrates the importance of focusing on vaccination strategies that increase the probability of eradication as close to 1 as possible, at which point the possibility of silent circulation becomes salient.
Revisiting a simple stochastic model by Eichner and Dietz (1996), we find that the dynamics associated with achieving eradication may significantly impact the probabiity of eradication and highlights that the starting point (or initial conditions) matter. For example, starting the model at equilibrium with 55% effective vaccination coverage with oral poliovirus vaccine (OPV) instead of at the pre-vaccine endemic equilibrium decreases the probability of eradication significantly. The greater force of infection from WPV at the endemic equilibrium compared with OPV vaccination with 55% coverage creates a rapid drop in population immunity when vaccination starts, which leads to a greater probability of reaching zero prevalence of WPV by chance.
Seasonality may also play an important role in the probability of eradication, with a short, intense high season followed by a relatively long low season increasing the probability significantly compared to an assumption of no seasonality.
In addition to large framing assumptions, varying individual inputs in the model can dramatically impact both the probability of eradication and the duration of circulation with no observed WPV cases.
The current certification criterion of requiring a period of at least 3 years of no observed paralytic polio cases detected by active, high quality acute flaccid paralysis surveillance to certify the absence of circulating WPVs appears reasonable in the context of the existing model results, but the analyses provide additional context and raise several issues for further consideration.

What are the study’s main recommendations?

One size does not fit all with respect to countries and poliovirus serotypes, and simple models may provide limited insights, which necessitates further efforts.
Serotype differences in the paralysis-to-infection ratios may imply significant differences between the relevant values of the observation time with no cases required to feel confident (within a specified level) that WPV circulation stopped and future efforts should further explore serotype differences.
Much of the success with respect to achieving relatively high levels of population immunity in some countries depended on conducting supplemental immunization activities, which differed from the conditions modeled, and this suggests that future modeling efforts will need to deal with a much more complex set of assumptions about the underlying population immunity and the actual interventions used to achieve global WPV eradication.
Starting at more realistic conditions with respect to population immunity may imply the need to wait longer to certify a population as polio-free in some places, and additional studies should focus on any real populations of concern with appropriate consideration of the actual conditions, heterogeneity, and more sophisticated assumptions about immunity (i.e., including waning and re-infection).
The choice of the confidence level that policy-makers require for certification comes with real trade-offs, which require additional and careful consideration.
Future studies will need to move away from the implicit simplifying assumption of perfect surveillance and recognize that surveillance quality may vary significantly, with poor-quality surveillance often coinciding with poverty, low vaccine coverage, and other conditions that favor poliovirus transmission.