PREPRINT: Temperature, humidity, and wind speed are associated with lower Covid-19 incidence

Despite a considerable amount of interest and hypothesis, empirical evidence on the association between Covid-19 and the meteorological factors (e.g., temperature, humidity) is lacking,1 many of which has been reported to be associated with chronic and infectious diseases including severe acute respiratory syndrome (SARS), and Middle East respiratory syndrome (MERS).2–5 In this study, we examined the association between (concurrent and historical) meteorological factors and the incidence of Covid-19 in 310 regions from 116 countries with ≥1 reported case by March 12, 2020.

Reported number of Covid-19 cases were drawn from the database developed by the Johns Hopkins University (data available at https://github.com/CSSEGISandData/COVID-19). We collated the concurrent and historical data since January 08, 2020 on temperature, relative humidity, wind speed, and ultraviolet (UV) index (data available at https://github.com/imantsm/COVID-19) – our primary meteorological factors of interest. We used multilevel mixed effects negative binomial regression model6 to examine the association between these factors and the incidence of Covid-19 with a two-level random intercepts nested within the regions (countries, or states/provinces for larger countries). We also conducted two additional analyses to examine the association between historical weather (preceding 1- and 2- week to coincide with the 2019-nCoV incubation period)7 and Covid-19 incidence. All the  models were adjusted for the regional and temporal trend and variability in the Covid-19 incidence, and other meteorological factors (such as columnar density of total atmospheric ozone, precipitation probability, sea-level air pressure, and daytime length). The effects estimates are reported as adjusted incidence rate ratio (IRR) and their corresponding 95% It is made available under a CC-BY-NC-ND 4.0 International license . author/funder, who has granted medRxiv a license to display the preprint in perpetuity. medRxiv preprint doi: https://doi.org/10.1101/2020.03.27.20045658. The copyright holder for this preprint (which was not peer-reviewed) is the 2 confidence interval (CI). Data analysis was conducted in Stata/SE 14.2 (StataCorp.
2015. College Station, TX: StataCorp LP).

Overall, the incidence of Covid-19 increased by 11% (adjusted IRR: 1.11, 95% CI: 1.10-11, P<0.001) per day after adjusting for all the variables listed above (dew point and cloud cover were initially considered, but later excluded due to multicollinearity). In the adjusted model, daily maximum temperature, relative humidity, and wind speed were associated with a lower incidence of Covid-19 (Table 1). There was an inverse association between Covid-19 incidence and 14-day lagged UV index (but not with the concurrent or the 7-day lagged data). To our knowledge, this is the first empirical study to comprehensively examine the association between the incidence of Covid-19 and a range of meteorological factors spanning across the
world. Our finding of an inverse relationship between temperature (and humidity) and the incidence of Covid-19 may suggest a cold and dry environment more favourable condition for virus survival, as was proposed for other coronavirus such as SARS-CoV and MERS-CoV.2–5 An inverse association with wind speed may indicate a shorter suspending time in the air due to
dilution and removal of Covid-19.3

An inverse association with a higher UV index would suggest viral destruction at higher temperature,3 but the association did not hold with the concurrent or 7-day UV index.

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