Seasonal thermal transition dynamics and influenza incidence across multiple cities

Seasonal influenza outbreaks show strong temporal regularity, but their magnitude and timing vary across years and locations. This study examined whether thermal transition dynamics, in addition to absolute temperature, are associated with influenza incidence across contrasting climatic settings. We conducted a retrospective multi-city analysis of Beijing, Tokyo, New York, London, and Sydney. City-specific climatological baselines were constructed from daily temperature records for 1965–2025 and compared with daily influenza incidence data for 2020–2025. Analyses considered long-term thermal seasonality, short-term temperature–incidence co-variation, temperature anomalies, daily temperature change (ΔT), and exploratory snowfall indicators. Descriptive analyses showed that increases in reported influenza incidence frequently coincided with rapid warming, delayed cooling, and larger departures from city-specific temperature baselines. In adjusted time-series analyses, ΔT remained associated with influenza incidence in several cities after accounting for seasonal structure and absolute temperature, although the direction and magnitude varied by city and lag period. Absolute temperature-anomaly magnitude showed less consistent independent associations, and the snowfall findings remained exploratory. Thermal transition rate and deviation from long-term climatic norms may provide complementary information for describing the timing of influenza activity beyond absolute temperature alone. Because the analysis was observational and covered the COVID-19 pandemic period, the findings should be interpreted as temporal associations rather than causal effects. Further studies using longer influenza time series and additional environmental and surveillance covariates are needed.