Linking temperature and humidity to influenza-like illness: A long-term time series study in the Netherlands, 1970–2016

Introduction Climatic parameters affect the survival and transmissibility of respiratory viruses causing influenza-like illness (ILI), like influenza, respiratory syncytial virus and rhinovirus. In temperate countries, ILI activity is inversely associated with humidity and temperature (which affect both the onset and size of ILI epidemics), and we hypothesized that changes in the average values of these climatic parameters (which have occurred in Europe in recent decades) may explain the declining trends in ILI incidence rates observed during the same period. Here, we tested this hypothesis by linking ILI and climatic time series in the Netherlands for the period 1970–2016. Methods Influenza-like illness surveillance data for the period 1970–2016 were obtained from the Netherlands Institute for Health Services Research (NIVEL), which relies on a network of 60 sentinel general practitioners in 40 practices that are distributed throughout the Netherlands (including urban and rural areas) and cover approximately 0.7% of the Dutch population. Data on daily average absolute humidity (g/m 3 ) and temperature for the weather station of De Bilt (which is located in the middle of the country) for the period 1970–2016 were obtained from the Royal Netherlands Meteorological Institute. Linear regression models were used to assess whether the season average ILI incidence rate changed over time, and whether there was any change over time in daily average absolute humidity and temperature. We modelled the association between climate and ILI activity through negative binomial regression models with the weekly ILI count as dependent variable, absolute humidity (or temperature) with up to three lagged weeks as independent variables, and covariates accounting for contagiousness of ILI-causing agents and presence of immunity. Model-based predictions were made using a leave-one-out cross-validation procedure. The study was funded by Sanofi-Pasteur. Results The season average ILI incidence rate declined significantly ( P P 3 were 0.965 (95% CI: 0.951–0.979, P -value  P -value  Conclusions Changes in absolute humidity and temperature were able to satisfactorily explain the long-term trends of ILI incidence rate in the Netherlands during 1970–2016. Temperate countries should monitor climate-related change in infectious diseases epidemiology so that measures to mitigate their impact can be planned and implemented.