Environmental risk factors of airborne viral transmission: Humidity, Influenza and SARS-CoV-2 in the Netherlands

ObjectiveThe relationship between specific humidity and influenza/SARS-CoV-2 in the Netherlands is evaluated over time and at regional level. DesignParametric and non-parametric correlation coefficients are calculated to quantify the relationship between humidity and influenza, using five years of weekly data. Bayesian spatio-temporal models--with a Poisson and a Gaussian likelihood--are estimated to find the relationship between regional humidity and the daily cases of SARS-CoV-2 in the municipalities and provinces of the Netherlands. ResultsAn inverse (negative) relationship is observed between specific humidity and the incidence of influenza between 2015 and 2019. The space-time analysis indicates that an increase of specific humidity of one gram of water vapor per kilogram of air (1 g/kg) is related to a reduction of approximately 5% in the risk of COVID-19 infections. ConclusionsThe increase in humidity during the outbreak of the SARS-CoV-2 in the Netherlands helped to reduce the risk of regional COVID-19 infections. Public policies that promote higher levels of specific humidification--above 6 g/Kg--can lead to significant reductions in the spread of respiratory viruses, such as influenza and SARS-CoV-2. Summary BoxO_ST_ABSWhat is already known on this subject?C_ST_ABSO_LIEnvironmental conditions have been related to the airborne transmission of respiratory viruses. C_LIO_LIPrevious observational studies have found an inverse correlation between humidity and the spread of SARS-CoV-2. C_LI What does this study add?O_LIWe analyzed the relation between specific humidity and airborne virus transmission using data with a higher temporal and spatial resolution. C_LIO_LISpatio-temporal risk estimates of SARS-CoV-2 are obtained after controlling for humidity levels at sub-national level in the Netherlands. C_LIO_LIOur results indicate that the increase of specific humidity during the outbreak of the SARS-CoV-2 helped to reduce the risk of regional COVID-19 cases in the Netherlands. Specifically, an increase of specific humidity of one gram of water vapor per kilogram of air (1 g/kg) is related to a reduction of approximately 5% in the risk of COVID-19 cases. C_LI