Association between the proportion of Plasmodium falciparum and Plasmodium vivax infections detected by passive surveillance and the magnitude of the asymptomatic reservoir in the community: A pooled analysis of paired health facility and community data

Background: Passively collected malaria case data are the foundation for public health decision-making. Because of population-level immunity, infections may not always be sufficiently symptomatic to prompt care-seeking. Understanding the proportion of all Plasmodium infections expected to be detected by the health system becomes particularly paramount in elimination settings. Our aim was to determine the relationship between the proportion of infections detected and transmission intensity for Plasmodium falciparum and P. vivax. Methods: The proportion of infections detected in routine malaria data [P(Detect)] was derived from paired household cross-sectional survey and routinely collected malaria data within health facilities. P(Detect) was estimated using a Bayesian model in 431 clusters spanning the Americas, Africa, and Asia. The association between P(Detect) and malaria prevalence was assessed using log-linear regression models. Changes in P(Detect) over time were evaluated using data from 13 time-points over two-years from The Gambia. Findings: The median estimated P(Detect) was 12.5% (Cluster IQR: 5.3 - 25.0%) and 10.1% (Cluster IQR: 5.0 - 18.3%) and decreased as the estimated log-PCR community prevalence increased (AOR: 0.63, 95% CI: 0.57 - 0.69 and AOR: 0.52, 95% CI: 0.47 - 0.57) for P. falciparum and P. vivax, respectively. Factors associated with improving P(Detect) included smaller catchment population size, high-transmission season, improved care-seeking behavior, and recent increases in transmission intensity. Interpretation: The proportion of all infections detected within health systems increases once transmission intensity is sufficiently low. The likely explanation for P. falciparum is that reduced exposure to infection leads to lower levels of protective immunity in the population increasing the likelihood that infected individuals will become symptomatic and seek care. In low transmission and pre-elimination settings, improving access to care and improving care-seeking behavior of infected individuals will lead to an increased proportion of infections detected in the community and may contribute to accelerating the interruption of transmission.