The role of older children and adults in wild poliovirus transmission

Abstract

As polio eradication inches closer, the absence of poliovirus circulation in most of the world and imperfect vaccination coverage are resulting in immunity gaps and polio outbreaks affecting adults. Furthermore, imperfect, waning intestinal immunity among older children and adults permits reinfection and poliovirus shedding, prompting calls to extend the age range of vaccination campaigns even in the absence of cases in these age groups. The success of such a strategy depends on the contribution to poliovirus transmission by older ages, which has not previously been estimated. We fit a mathematical model of poliovirus transmission to time series data from two large outbreaks that affected adults (Tajikistan 2010, Republic of Congo 2010) using maximum-likelihood estimation based on iterated particle-filtering methods. In Tajikistan, the contribution of unvaccinated older children and adults to transmission was minimal despite a significant number of cases in these age groups [reproduction number, R = 0.46 (95% confidence interval, 0.42-0.52) for >5-y-olds compared to 2.18 (2.06-2.45) for 0- to 5-y-olds]. In contrast, in the Republic of Congo, the contribution of older children and adults was significant [R = 1.85 (1.83-4.00)], perhaps reflecting sanitary and socioeconomic variables favoring efficient virus transmission. In neither setting was there evidence for a significant role of imperfect intestinal immunity in the transmission of poliovirus. Bringing the immunization response to the Tajikistan outbreak forward by 2 wk would have prevented an additional 130 cases (21%), highlighting the importance of early outbreak detection and response.

Keywords: epidemiology; infectious diseases; mathematical modeling.

Fig. 1.

The 2010 poliomyelitis outbreak in Tajikistan. (A) Geographic distribution of poliomyelitis cases plotted by district and colored according to their age group. The total population size for each district is indicated by the shading. Within a district, dots are randomly placed. (B) The age distribution of reported poliomyelitis cases by week of onset of paralysis. (C) Number of cases of poliomyelitis by week of onset of paralysis by age group (bars) with the median (blue line) and interquartile range (blue shading) of simulations under the best-fit transmission model, conditional on a major epidemic. The median (orange line) and interquartile range (orange shading) for simulations in the absence of a vaccination response are also shown. (D) Predicted number of cases prevented by the age-targeted vaccination response and under alternative scenarios (bars, median; error bars, interquartile range).

Fig. 2.

The 2010–2011 poliomyelitis outbreak in the Republic of Congo. (A) Geographic distribution of poliomyelitis cases plotted by district and colored according to their age group. The total population size for each district is indicated by the shading. Within a district, dots are randomly placed. (B) The age distribution of reported poliomyelitis cases by week of onset of paralysis. (C) Number of cases of poliomyelitis by week of onset of paralysis for each age group (bars) with the median (blue line) and interquartile range (blue shading) of simulations under the best-fit transmission model, conditional on a major epidemic. The expected outbreak dynamics in the absence of a response are not shown (compare with Fig. 1) because of uncertainty in the estimated vaccine efficacy.