Silent circulation of poliovirus in small populations

Abstract

BackgroundSmall populations that have been isolated by conflict make vaccination and surveillance difficult, threatening polio eradication. Silent circulation is caused by asymptomatic infections. It is currently not clear whether the dynamics of waning immunity also influence the risk of silent circulation in the absence of vaccination. Such circulation can, nevertheless, be present following a declaration of elimination as a result of inadequate acute flaccid paralysis surveillance (AFPS) or environmental surveillance (ES).MethodsWe have constructed a stochastic model to understand how stochastic effects alter the ability of small populations to sustain virus circulation in the absence of vaccination. We analyzed how the stochastic process determinants of the duration of silent circulation that could have been detected by ES were affected by R0, waning dynamics, population size, and AFPS sensitivity in a discrete individual stochastic model with homogeneous contagiousness and random mixing. We measured the duration of silent circulation both by the interval between detected acute flaccid paralysis (AFP) cases and the duration of circulation until elimination.ResultsAs R0 increased and population size increased, the interval between detected AFP cases and the duration of circulation until elimination increased. As AFPS detection rates decreased, the interval between detected AFP cases increased. There was up to a 22% chance of silent circulation lasting for more than 3 years with 100% AFP detection. The duration of silent circulation was not affected by the waning immunity dynamics.ConclusionWe demonstrated that small populations have the potential to sustain prolonged silent circulation. Surveillance in these areas should be intensified before declaring elimination. To further validate these conclusions, it is necessary to realistically relax the simplifying assumptions about mixing and waning.

Another look at silent circulation of poliovirus in small populations

“…These models generally agree that in most realistic situations, 3 years without any detected cases implies at least 95% confidence about no circulation, although our studies show that results depend on surveillance quality, serotype, seasonality, and vaccination strategies (Kalkowska, Duintjer Tebbens, Pallansch, et al., 2015; Kalkowska et al., 2012). In contrast to the prior literature, Vallejo et al. (2017) prominently reported (in their abstract) a 22% chance of at least 3 years between successive polio cases in a population of 10,000 people despite a 100% case detection rate.…”

“…Motivated by the re-emergence in Borno, a recent study used a stochastic model to examine whether simple, small, hypothetical populations not reached at all by vaccination can perpetuate WPV transmission while experiencing very long intervals between polio cases (Vallejo, Keesling, Koopman, & Singer, 2017). The potential for silent circulation influences the confidence about the interruption of poliovirus transmission as a function of time after the last detected poliovirus, which informs the decision to certify wild poliovirus eradication in a country, region, and globally.…”

“…Based on epidemiological experience and modeling, certifying a region or the world as polio-free requires no observed polio cases for at least three years (Debanne & Rowland, 1998; Eichner & Dietz, 1996; Kalkowska, Duintjer Tebbens, Pallansch, et al., 2015; World Health Organization, 2017). Prior to Vallejo et al. (2017), numerous modeling studies addressed the possibility of prolonged poliovirus circulation without any detected polio cases in different populations using different metrics, and spanning different model structures and assumptions about surveillance, vaccination, transmissibility, and demographics (Berchenko et al., 2017; Eichner & Dietz, 1996; Famulare, 2016; Houy, 2015; Kalkowska, Duintjer Tebbens, Pallansch, et al., 2015; Kalkowska, Duintjer Tebbens, & Thompson, 2012).…”

Possible neurogenic disorder in a female buried in the monastic cemetery at Ghazali (ca. 670–1270 CE), northern Sudan

“…Although overcrowding associated with urban growth, as well as ensuing sanitation issues, has been determined conducive to polio virus transmission, polio cases have been found both in urban and rural settings (Chaturvedi et al, ; Maru, Getahun, & Hoshna, ). Contemporary epidemiological studies have shown that small populations have the potential to sustain prolonged silent circulation (i.e., asymptomatic spread of polio), with an upper bound of 15 years between two successive paralytic cases (Vallejo, Keesling, Koopman, & Singer, ). On the other hand, computer modelling suggests that isolated subpopulations of less than 4,000 cannot sustain endogenous transmission of wild polio virus for more than 5 years (Duintjer Tebbens, Kalkowska, & Thompson, ).…”

The game is not over for polio virus in India; it can strike back again

“…Currently annual cases of AFP is greater than 50,000 then what is the meaning of Polio free country? 4 In India, Uttar Pradesh and other states, disease and poverty are the main hurdle in polio vaccination than political, religious, culture. 5 As per report of Times of India, minimum three polio vaccines batches which contain 1.5 lakh vials reported contaminated with polio virus type 2. WHO and the Ministry of Health started surveillance, especially in Uttar Pradesh, Telangana and Maharashtra where cases of contaminated vaccines reported at large level.…”