The potential impact of expanding target age groups for polio immunization campaigns

BMC Infectious Diseases The potential impact of expanding target age groups for polio immunization campaigns Radboud J Duintjer Tebbens 2 Dominika A Kalkowska 1 2 Steven GF Wassilak 0 Mark A Pallansch 4 Stephen L Cochi 0 Kimberly M Thompson 2 3 0 Global Immunization Division, Center for Global Health, Centers for Disease Control and Prevention , Atlanta, GA , USA 1 Delft Institute of Applied Mathematics, Delft University of Technology , Delft , The Netherlands 2 Kid Risk, Inc , 10524 Moss Park Road, Site 204-364, Orlando, FL 32832 , USA 3 University of Central Florida, College of Medicine , Orlando, FL , USA 4 Division of Viral Diseases, National Center for Immunization and Respiratory Diseases, Centers for Disease Control and Prevention , Atlanta, GA , USA Background: Global efforts to eradicate wild polioviruses (WPVs) continue to face challenges due to uninterrupted endemic WPV transmission in three countries and importation-related outbreaks into previously polio-free countries. We explore the potential role of including older children and adults in supplemental immunization activities (SIAs) to more rapidly increase population immunity and prevent or stop transmission. Methods: We use a differential equation-based dynamic poliovirus transmission model to analyze the epidemiological impact and vaccine resource implications of expanding target age groups in SIAs. We explore the use of older age groups in SIAs for three situations: alternative responses to the 2010 outbreak in Tajikistan, retrospective examination of elimination in two high-risk states in northern India, and prospective and retrospective strategies to accelerate elimination in endemic northwestern Nigeria. Our model recognizes the ability of individuals with waned mucosal immunity (i.e., immunity from a historical live poliovirus infection) to become re-infected and contribute to transmission to a limited extent. Results: SIAs involving expanded age groups reduce overall caseloads, decrease transmission, and generally lead to a small reduction in the time to achieve WPV elimination. Analysis of preventive expanded age group SIAs in Tajikistan or prior to type-specific surges in incidence in high-risk areas of India and Nigeria showed the greatest potential benefits of expanded age groups. Analysis of expanded age group SIAs in outbreak situations or to accelerate the interruption of endemic transmission showed relatively less benefit, largely due to the circulation of WPV reaching individuals sooner or more effectively than the SIAs. The India and Nigeria results depend strongly on how well SIAs involving expanded age groups reach relatively isolated subpopulations that sustain clusters of susceptible children, which we assume play a key role in persistent endemic WPV transmission in these areas. Conclusions: This study suggests the need to carefully consider the epidemiological situation in the context of decisions to use expanded age group SIAs. Subpopulations of susceptible individuals may independently sustain transmission, which will reduce the overall benefits associated with using expanded age group SIAs to increase population immunity to a sufficiently high level to stop transmission and reduce the incidence of paralytic cases. Polio; Eradication; Dynamic modeling; Disease outbreaks - Background The global commitment to eradicate wild polioviruses (WPVs) and end poliomyelitis led to the launch of the Global Polio Eradication Initiative (GPEI) in 1988, which successfully reduced global polio incidence by 99% [1] and eradicated one of the three WPV serotypes (i.e., type 2 or WPV2) around 1999 [2]. By early 2014, all except three countries (i.e., Afghanistan, Nigeria and Pakistan) * Correspondence: 1Kid Risk, Inc, 10524 Moss Park Road, Site 204-364, Orlando, FL 32832, USA Full list of author information is available at the end of the article successfully interrupted indigenous transmission of WPV type 1 (WPV1) and no country has reported a case of WPV type 3 (WPV3) for over a year [3]. Despite these successes, the GPEI still needs to stop transmission of WPV1 everywhere contemporaneously and transition away from the use of oral poliovirus vaccine (OPV) to achieve the ultimate goal of ending all cases of poliomyelitis [4]. Partly due to importations into previously polio-free countries that caused outbreaks, the global annual incidence of paralytic polio remained around 1,000-2,000 cases between 2000 through 2010. The World Health Assembly in 2012 urged further intensification of the GPEI by declaring the completion of global poliovirus eradication a programmatic emergency for global public health ([5], p. 2). During 2012 the GPEI reported the fewest cases and smallest number of countries reporting cases in its history [6]. Successful polio eradication requires achieving and maintaining sustained high levels of population immunity until wild poliovirus transmission stops everywhere contemporaneously [4]. Although many high- and middle-income countries can maintain high population immunity by reaching and sustaining high routine immunization coverage, other countries must rely on the use of supplemental immunization activities (SIAs) that provide OPV during a short period of time regardless of immunization history to periodically and significantly boost population immunity [6]. Historically, SIAs emerged as a strategy to interrupt poliovirus transmission during the low season by flooding the environment with vaccine virus so that a susceptible child has a lower probability of encountering wild virus and/or by rapidly increasing population immunity so that fewer susceptible children remain ([7], p. 1332). The GPEI continues to rely heavily on SIAs to close immunity gaps resulting from sub-optimal routine immunization by targeting all children under 5 years of age [8]. Typically, polio SIAs focus on immunizing children younger than 5 years of age, because infants become susceptible once they lose protection from maternal antibodies and most people get exposed to circulating viruses during early childhood. Based on experience dating back to early OPV trials by Albert Sabin [9] and polio elimination from the Western Hemisphere [10-12], SIAs can dramatically reduce the prevalence of WPV and rapidly interrupt transmission. However, following the introduction of vaccination, the circulation of viruses may become more episodic, and those missed by vaccination may continue to remain susceptible and begin to accumulate. With the passage of time since the interruption of WPV transmission in most places, importation outbreaks increasingly involve paralytic patients 5 or more years of age [13-17], which provides clear evidence that immunity gaps persist beyond early childhood in polio-free areas. Recognizing the need to protect older children and adults in these situations, the GPEI modified its outbreak response recommendations to explicitly expand the age group targeted in the first two SIAs up to (...truncated)