For more than three decades, humankind has pursued the possibility of a “polio-free world.”1 However, outbreaks following wild poliovirus (WPV) importations into previously polio-free countries remain an ongoing risk; this risk disappears only when polio is eradicated.2 Even after gaining polio-free certification, countries have struggled to remain polio-free. In 2010, the first WPV importation into the European region since the region was declared polio-free in 2002 resulted in 476 confirmed cases: 458 in Tajikistan, 14 in Russia, three in Turkmenistan, and one in Kazakhstan. In Africa and Asia, 11 new importations into six countries were observed in 2010; 30 WPV importations during 2008–2009 resulted in 215 WPV cases in 15 African countries during 2009–2013. Across six African and Asian countries, 11 new importations were recorded in 2009–2010.3 Kenya and Somalia have not been spared from the impact of WPV importation.4 In Somalia, the first importation occurred during 2005–2007, resulting in more than 200 cases of paralytic polio.5 Likewise, in Kenya, two cases in Garissa County were reported in 2006, 19 cases in Turkana County in 2009, and one case in Rongo district in 2011. Somalia experienced a polio-free period from 2007 to 2013.5 This period of calm was upset when an outbreak of WPV type 1 (WPV1) rattled the Horn of Africa (HOA).6 In May 2013, the Somalia Ministry of Health (MOH) and the WHO reported a confirmed WPV1 case in a child from Mogadishu (Banadir region). Subsequently, in May 2013, eight additional WPV1 cases were confirmed in Somalia: seven in the Banadir region and one in the Bay region.7 Just 3 weeks after the initial polio cases were detected in May 2013 in Somalia, Kenya reported its first case across the border in the Dadaab refugee camp. As a result, five people were paralyzed by polio, including young adults, in the northeastern part of Kenya.8 Polio quickly spread from Somalia to its neighboring countries of Ethiopia and Kenya. By April 2013, the case count stood at 223: 199 cases in Somalia, 14 in Kenya, and 10 in Ethiopia. All the Somalia polio cases belonged to cluster N5A,† which was known to have been circulating in northern Nigeria since 2011. The 2013 HOA outbreak vividly confirmed that “All countries will continue to have some level of risk for WPV outbreaks as long as endemic circulation continues in Afghanistan, Nigeria, and Pakistan.”9 At the same time, the Global Polio Eradication Initiative entered a new phase, with a significant reduction in cases in endemic countries and a heightened recognition of the risk for the international spread of the virus.10 To combat the threat of an international outbreak, the WHO declared polio a public health emergency of international concern in May 2014 and issued recommendations requiring proof of polio vaccination for travel to and from countries experiencing polio cases.10 Frequent cross-border movement of the high-risk mobile populations between Kenya and Somalia and the low level of population immunity in the region continue to be major contributing factors to the spread of poliovirus and the risk of ongoing transmission.11 In 2014, Kenya’s MOH requested the United States Agency for International Development (USAID)-funded CORE Group Polio Project (CGPP) Kenya and Somalia HOA Secretariat based in Nairobi, Kenya, to initiate polio eradication activities in five counties along the Kenya–Somalia border deemed at high risk for poliovirus importation. It was clear that effective immunization activities across borders and migration pathways were essential to improve immunization rates. Under the leadership of the respective MOHs of Kenya and Somalia and in collaboration with the WHO, the CGPP began holding cross-border meetings in October 2014—a significant and instrumental move that would shape a systematic, unified, and well-coordinated response in the form of the Cross-Border Health Initiative (CBHI).12 Objectives arising from the 2015 cross-border meetings targeted improving collaboration between the health and administrative authorities of border regions through enhancing acute flaccid paralysis (AFP) surveillance sensitivity, increasing coverage of supplemental immunization activities (SIAs), and improving coverage and access to quality routine immunization services in the HOA border regions. Before the formation of the CBHI, cross-border committee meetings were first initiated by the WHO in collaboration with the Intergovernmental Authority on Development (IGAD) under the “Health for Peace Initiative” in 1996. However, these cross-border committees were formed in only a few selected sites and the meetings were ad hoc, resulting in limited capacity for implementation, monitoring, accountability, resource allocation, and sustainability of cross-border polio eradication activities. To address these gaps yet keeping and reaffirming the same set of valuable original objectives, the CGPP Kenya and Somalia HOA Secretariat over a 1-year period transformed the ad hoc cross-border meetings into a full CBHI in October 2015. The secretariat subsequently established more CBHI committees in Kenya and Somalia. The work of the committees funded by the CGPP set the course for the eventual full implementation of the CBHI. The overarching goal of the CBHI is to reach every child with polio vaccine.13 Cross-border coordination bridges the disease surveillance gaps by forming partnerships among institutions, agencies, and communities in cross-border areas. Specifically, the CBHI works to ensure the vaccination of all cross-border populations, to support the detection of cases of AFP, to conduct joint case investigations of transborder AFP and WPV cases, and to synchronize all polio SIAs. The Cross-Border Health Initiative. In October 2015, the CGPP established a total of seven CBHI committees in five polio high-risk counties‡ in Kenya and two regions of Somalia. Results of a risk assessment identified Turkana, Garissa, Wajir, Marsabit, and Mandera counties in Kenya for inclusion in the CBHI; all share a border with either South Sudan, Somalia, Ethiopia, or Uganda’s northern region. Based on the risk assessment, the CGPP established more committees in Kenya’s 14 subcounties bordering Somalia and Ethiopia§; in Somalia, the CGPP selected six districts from the Gedo and Lower Juba regions‖ for committee work. The MOH officials from the respective governments provided the leadership for the establishment of the CBHI and the formation of the committees, with funding from the CGPP. In Kenya, the government-led committees consisted of representatives from the five counties and subcounties. At the county level, the representatives included the director for health, the disease surveillance coordinator, the Expanded Program on Immunization (EPI) coordinator, the health records and information officer, and the community health strategy focal person. At the subcounty level, representation included subcounty coordinators and disease surveillance or EPI officers. In addition, the CGPP implementing partner officers, WHO and United Nations Children’s Fund (UNICEF) county coordinators, and officers from border administration, immigration, and security completed the committee membership. In Somalia, the committees consisted of regional and district medical officers and a regional EPI coordinator. Similar to the Kenya committees, representation included the CGPP implementing partner officers, WHO regional and district coordinators, UNICEF field staff, and officers from border administration, immigration, and security. To reach high-risk mobile populations, the CGPP, in collaboration with the local authorities and CBHI, identified and profiled both formal and informal crossing points and communities at borders, transit hubs, and migratory routes. This exercise was beneficial; after developing detailed micro-planning to identify and document all border towns, villages, and settlements and their inhabitants, it was possible to estimate the number of children in the catchment areas. Moreover, the major transit points in the area provided pertinent information about population movement patterns, whereas joint mapping of border crossing points and border communities served to improve micro-planning for country-specific and joint country cross-border activities. The CGPP’s non-governmental organization (NGO) implementing partners and local health authorities mapped 11 border health facilities, 161 formal/informal border crossing points, and 372 villages along the border as shown in Table 1. The estimated population of children younger than 5 years is 557,036 and of children younger than 1 year is 120,068. Table 1 Mapping border crossing points, border villages, and population sizes