Abstract
Vaccination programs have averted millions of childhood deaths, yet vaccine-preventable diseases (VPDs) continue to resurge as coverage declines and pathogen evolution undermines previously successful vaccines. Anticipating resurgence is a public health priority. We review theoretical and empirical advances in the study of early warning signals (EWS) of epidemic transitions, with a focus on critical slowing down (CSD) - a phenomenon in which recovery from perturbations becomes slower near the epidemic threshold. We summarize the mechanisms that generate CSD, indicators that can be extracted from surveillance data, and the conditions under which signals may be detectable. We then examine case studies to illustrate the opportunities and challenges of applying EWS to VPD resurgence. Theory and computer simulations show that CSD can precede both elimination and resurgence, with increases in variance and autocorrelation calculated from disease surveillance reports emerging as consistent indicators. Empirical evidence supports this potential, though performance depends on noise structure, seasonality, spatial clustering, and outbreak responses. Case studies highlight both successful applications and contexts where signals were weak or absent. EWS offer a promising framework for anticipating VPD resurgence, but further research is required to refine methods, integrate mechanistic and social-behavioral drivers, and evaluate applicability across pathogens and settings.