PROJECT SUMMARY Influenza is a persistent viral disease worldwide, with high case counts leading to deaths and losses in productivity yearly. In temperate regions of the world, such as the United States, influenza appears in annual wintertime cycles, with the influences of changing seasons creating predictable patterns of incidence, despite the fact that different subtypes and strains of the virus predominate during each season. In tropical regions, distinct seasonal patterns are not well established for influenza, suggesting that annual or climatic forces may not dictate transmission in these areas. The tropics include many resource-limited areas of the world where respiratory disease research is underfunded and influenza vaccination is not prioritized. Transmission patterns of influenza can be described through the use of mathematical models, which incorporate known biological mechanisms, such as infectivity, recovery, and duration of immunity following infection, in order to describe the dynamics, or population-level behaviors, of the virus. Models can incorporate cyclic patterns of influenza dynamics, which can include both seasonal cycles aligning with a calendar year as well as nonannual periodic cycles; models can also incorporate irregular epidemics, though this is more challenging to model and less commonly seen in current literature. Irregular epidemics can be viewed as “regime switches,” which describes transitions between endemic time periods with consistently low incidence and epidemic time periods with short periods of elevated incidence. This study aims to investigate potentially cyclic or acyclic dynamics of influenza in the tropics, using Vietnam as a case study. The first aim seeks to define patterns describing cyclic or acyclic dynamics and then use a mathematical model to show under what conditions acyclic dynamics can exist. Influenza characteristics that could allow acyclic dynamics potentially include duration of immunity following infection, cross-immunity, and population structure. The second aim seeks to use the developed model as well as influenza data from Vietnam to determine whether influenza dynamics follow cyclic or acyclic patterns. Comparing these results to identical analyses using data from temperate regions influenza data will allow us to compare the strength of annual cycles between tropical and temperate regions. The third aim seeks to identify characteristics of influenza virus, population patterns, or external factors that may be able to explain the cyclic or acyclic patterns observed in influenza dynamics in Vietnam. This will show what characteristics of influenza or external factors provide the strongest predictions of future influenza incidence in Vietnam. Uses of the study results include better informed and appropriately-timed vaccination strategies for influenza in Vietnam. The main points of novelty in this study lie in the explicit testing for cyclic or acyclic patterns in the tropics, which is le...