Prevalence of Asymptomatic Poliovirus Infection in Older Children and Adults in Northern India: Analysis of Contact and Enhanced Community Surveillance, 2009

In 1988, the World Health Assembly resolved to eradicate polio worldwide [1]. In that year, the estimated number of paralytic cases was 350 000 in 125 countries. In 2009 there were <2000 cases of paralytic polio detected worldwide, and in 2012 only 217 cases were reported from the 3 remaining polio-endemic countries (Nigeria, Pakistan, and Afghanistan); an additional 5 cases were detected in Chad, the last remaining country with reestablished wild poliovirus (WPV) transmission, and 1 case, imported from Nigeria, was detected in Niger [2, 3]. WPV has 3 serotypes (1, 2, and 3). WPV2 has been successfully eradicated, with the last cases reported in 1999 [4]. In 2012, 88% of all reported cases of paralytic polio were due to WPV1 [2]. India was a polio-endemic country until 2011, with the last paralytic case of polio detected in January 2011 [2, 5]. This remarkable success is attributed to the Government of India and their local and international partners, who have been investing significant human and financial resources into polio eradication for many years. The main strategies of the India polio eradication program have been to focus on high-quality supplemental immunization activities (SIAs) targeting children from birth to 5 years of age with oral poliovirus vaccine (OPV), a highly developed social mobilization strategy and approach to reach millions of migrants with SIAs, a well-performing acute flaccid paralysis (AFP) surveillance system, and strengthening of routine immunization. Prior to the successful elimination of the last foci of WPV transmission in 2011, India had prolonged low-level WPV circulation, as well as occasional outbreaks of both WPV1 and WPV3 concentrated mainly in the northern states of Uttar Pradesh and Bihar [6–12]. The introduction, in 2005, of more-efficient monovalent and, later, bivalent OPV vaccines into SIAs was considered to be the main innovation leading to successful eradication of WPV from India [13–15]. During 2005–2011, SIAs were conducted practically every month in the high-risk areas of western Uttar Pradesh and Bihar; despite the high quality of these SIAs, it took >6 years of intense efforts and enormous resources to rid India of WPV. The strategy to target children <5 years of age in SIAs was first successfully implemented in the Americas in the 1980s and was later adopted in other regions [16]. This strategy is based on empirical data of the age distribution of paralytic cases and the assumption that older children and adults are mostly immune to infection. Older individuals who do get infected are mostly asymptomatic, and they are also assumed to have limited ability to transmit virus to others, because of behavioral factors, such as good personal hygiene, because of a shorter duration and lower load of poliovirus shedding in stool, compared with younger individuals [17–26]. The operating premise of the global polio eradication program is that interrupting transmission in young children is sufficient to achieve poliovirus eradication. While the obvious success of eliminating poliovirus circulation in India supports this strategy, it does not address whether alternative approaches would have achieved the same goal more quickly and, possibly, more cost-effectively [16]. In India in 2009, however, the assumption that immunizing only children 98%), and yet low-level circulation of WPV1 continued [15]. Reinfection and shedding of infectious virus in immune individuals has been well documented, as has the fact that enteric mucosal immunity wanes with time [30, 31]. Although young children who are routinely targeted by SIAs have the opportunity to have their mucosal immunity boosted, those outside the target age group will lose mucosal immunity over time. Because these individuals represent the majority of the population, even small individual contributions to virus shedding could, in aggregate, contribute to virus circulation. The relative contribution of different age groups and different immunity status to virus circulation remains very difficult to measure. In response to the persistence of WPV transmission in northern India, enhanced surveillance for WPV was temporarily set up in 2009 in the 2 Indian regions where the incidence of poliovirus infection was greatest, western Uttar Pradesh and the Kosi river area of Bihar, to assess the potential role of older individuals in WPV circulation by measuring the rate of WPV shedding in different target populations. The surveillance data from Uttar Pradesh were obtained from investigations of household and close neighborhood contacts of patients with confirmed poliomyelitis (Uttar Pradesh Contact Surveillance); the data from Bihar were obtained from community surveillance (Bihar Community Surveillance). This report summarizes the epidemiologic and virologic data from the enhanced surveillance.