The role of older children and adults in wild poliovirus transmission.

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.

Polio eradication in the World Health Organization African Region, 2008-2012.

A renewed commitment at the regional and the global levels led to substantial progress in the fight for polio eradication in the African Region (AFR) of the World Health Organization (WHO) during 2008-2012. In 2008, there were 912 reported cases of wild poliovirus (WPV) infection in 12 countries in the region. This number had been reduced to 128 cases in 3 countries in 2012, of which 122 were in Nigeria, the only remaining country with endemic circulation of WPV in AFR. During 2008-2012, circulation apparently ceased in the 3 AFR countries with reestablished WPV transmission-Angola, the Democratic Republic of the Congo, and Chad. Outbreaks in West Africa continued to occur in 2008-2010 but were more rapidly contained, with fewer cases than during earlier years. This progress has been attributed to better implementation of core strategies, increased accountability, and implementation of innovative approaches. During this period, routine coverage with 3 doses of oral polio vaccine in AFR, as measured by WHO-United Nations Children's Fund estimates, increased slightly, from 72% to 74%. Despite this progress, challenges persist in AFR, and 2013 was marked by new setbacks and importations. High population immunity and strong surveillance are essential to sustain progress and assure that AFR reaches its goal of eradicating WPV.

An outbreak of wild poliovirus in the Republic of Congo, 2010-2011.

BACKGROUND: The Republic of Congo has had no cases of wild poliovirus type 1 (WPV1) since 2000. In October 2010, a neurologist noted an abnormal number of cases of acute flaccid paralysis (AFP) among adults, which were later confirmed to be caused by WPV1. METHODS: Those presenting with AFP underwent clinical history, physical examination, and clinical specimen collection to determine if they had polio. AFP cases were classified as laboratory-confirmed, clinical, or nonpolio AFP. Epidemiologic features of the outbreak were analyzed. RESULTS: From 19 September 2010 to 22 January 2011, 445 cases of WPV1 were reported in the Republic of Congo; 390 cases were from Pointe Noire. Overall, 331 cases were among adults; 378 cases were clinically confirmed, and 64 cases were laboratory confirmed. The case-fatality ratio (CFR) was 43%. Epidemiologic characteristics differed among polio cases reported in Pointe Noire and cases reported in the rest of the Republic of Congo, including age distribution and CFR. The outbreak stopped after multiple vaccination rounds with oral poliovirus vaccine, which targeted the entire population. CONCLUSIONS: This outbreak underscores the need to maintain high vaccination coverage to prevent outbreaks, the need to maintain timely high-quality surveillance to rapidly identify and respond to any potential cases before an outbreak escalates, and the need to perform ongoing risk assessments of immunity gaps in polio-free countries.