Where did the nets go? – Tracing bed net migration on Bioko Island

Background. Long-lasting insecticidal nets (LLINs) are commonly distributed through mass distribution campaigns (MDCs). MDCs are commonly followed by substantial loss of LLINs, affecting coverage. Here, data from an MDC and a malaria indicator survey (MIS) on Bioko Island were used to characterize LLIN mobility. Methods. Between October 2014 and July 2015, an MDC reached most of the households of the island. Distributed nets were marked with a unique code corresponding to the community where they were distributed. An MIS conducted in August and September 2015 allowed to measure LLIN loss and mobility by verifying the number of existing nets and their community code. Multivariate models identified factors associated with LLINs with mismatched codes. A source-sink analysis measuring risk gradient between the source community and the sink household was used to assess the overall effect of migrating nets. Results. The MIS revealed that 40.4% of LLINs distributed during the MDC had been lost. Among the surviving nets (7,393/12,397), community codes were visible only for 3,413 (46.2%), and 551 (16.1%) revealed a mismatched code. The factor most strongly associated with mismatched LLINs was open eaves (OR: 1.88; 95% CI: 1.48 – 2.38, p < 0.001). The source sink analysis showed that while the majority of LLINs migrated from lower to higher risk areas (62.3%), almost a third migrated in the opposite direction. Conclusion. While redistribution had a positive impact on some communities by serving populations at higher risk of transmission, many households in high-risk areas lost their nets to areas with lower risk. This, coupled with the significant LLIN loss post-MDC, pointed to critical inefficiencies and prompted a shift in LLIN distribution strategy from MDC to demand-driven fixed distribution points.