Why vaccine models keep getting the math wrong

Mathematical models of disease spread typically plug vaccine efficacy numbers directly into their equations, but this ignores a crucial distinction: whether a vaccine blocks infection entirely or just reduces its severity. When vaccines provide "leaky" protection—lowering but not eliminating infection risk—standard estimates from clinical trials don't match the parameters models actually need. The authors show this mismatch causes models to underestimate vaccination's real-world impact and miscalculate herd immunity thresholds, then provide a correction method for more accurate public health projections.