To enhance Sierra Leone's polio surveillance capacity, a team of Environmental Surveillance (ES) experts from WHO Regional Office for Africa visited the country to support the Ministry of Health and the National Public Health Agency (NPHA). The mission focused on improving the performance of Environmental Surveillance (ES) sites in Freetown, considering expansions to other towns, and building the capacity of local sample collectors and supervisors. The ongoing cVDPV2 (circulating vaccine-derived poliovirus type 2) outbreak in Sierra Leone and neighboring countries like Liberia and Guinea, coupled with the detection of 15 isolates across four of the five existing ES sites, highlighted the urgent need for enhanced surveillance. This prompted WHO's expert visit to strengthen the country’s ES system and complement its Acute Flaccid Paralysis (AFP) surveillance system. 

Over a four-day period, the team conducted site assessments and reviewed surveillance data. Although 80% of the sites met global virus detection targets, challenges such as delayed sample shipments and insufficient supplies were noted. Due to the high population density in some areas, the team also explored potential expansions to high-risk regions such as Bo and Kenema. While no suitable site was identified in Kenema, the Shenge Market site in Bo was selected for future expansion. Additionally, 21 participants, including sample collectors and supervisors, received refresher training focused on improving transportation logistics, revising sampling schedules, and using advanced electronic data tools. These measures are essential for making the ES system more responsive to polio threats, which continue to affect the Mano River Union countries.

In Sierra Leone, sentinel sites are concentrated in Western Area Districts due to the region’s high population density, especially in Freetown. This urban environment increases the risk of disease outbreaks, requiring enhanced surveillance. Additionally, key healthcare facilities and resources are located in this area, facilitating effective monitoring and response to health threats. The ES system detects poliovirus in sewage, offering an early warning by identifying virus particles shed in the feces of infected individuals. Expanding ES to regions like Bo will broaden the country’s surveillance coverage and improve the ability to detect and respond to potential outbreaks. 

The ES system, established in 2021 following the detection of cVDPV2 in 2020, is central to Sierra Leone’s polio eradication strategy. Initially, sample collection was conducted weekly, but later transitioned to bi-weekly and monthly intervals. However, after the detection of cVDPV2 at the Mabella Sawmill Bridge site in March 2024, bi-weekly collections were reinstated. 

The Environmental Surveillance Optimization Mission laid a strong foundation for improving Sierra Leone’s ability to detect and respond to poliovirus. Key next steps include ensuring that the newly identified ES site in Bo becomes fully operational with ongoing training and logistical support, enhancing the sample shipment process to ensure timely deliveries, appointing dedicated supervisors to underperforming sites, improving documentation, and labeling with preprinted stickers and triplicate ES sample forms, and providing essential supplies such as personal protective equipment (PPE) to all sites. Addressing these areas will better equip Sierra Leone to detect and respond to poliovirus, contributing to global polio eradication efforts.

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