COVID-19: Associations of climate and public health interventions

Associations of climate and public health interventions.

Heneghan C, Jefferson T.

Published on June 15, 2020

Reference Jüni P, Rothenbühler M, Bobos P et al. Impact of climate and public health interventions on the COVID-19 pandemic: a prospective cohort study. CMAJ May 25, 2020 192 (21) E566-E573 2020
Study type
Country 144 geopolitical areas worldwide
Funding Details This research was completed, in part, with funding from the Canada Research Chairs Program and the Canadian Institutes of Health Research.
Transmission mode Meteorological
Exposures Latitude, temperature, humidity, school closures, restrictions of mass gatherings, and measures of social distancing.

Bottom Line

Transmission of COVID-19 was associated with weakly associated with relative or absolute humidity and not associated with latitude and temperature.

Evidence Summary

For every 10% increase in relative humidity, there was a 9% relative reduction in epidemic growth (95% CI, 4% to 15%). For absolute humidity per 5 g/m3 increase there was a relative reduction of 8% (95% CI, 1% to 15%).

Restrictions in mass gatherings led to a 35% relative reduction in epidemic growth (95% CI, 21% to 48%), school closures 37% (95% CI, 48% to 22%) and measures of social distancing a 38% relative reduction in epidemic growth (95% CI,15% to  55%),

There were no associations found for latitude and temperature. In a multivariable model, the association with a number of implemented public health interventions remained significant (p for trend = 0.001), whereas the association with absolute humidity was not significant.

What did they do?

The study included 144 geopolitical areas with at least 10 cases as of March 20, 2020 (total 375, 609 cases) in the analyses. Most areas were in the northern hemisphere, near sea level and had temperate climates. (China was excluded as its epidemic growth had decelerated).

The exposure period was prespecified to last from March 7 to March 13, 2020. Data on latitude, temperature and humidity were collected for the capital of each area. Data on school closures, restrictions of mass gatherings and social distancing were collected at the level of the geopolitical area.

The outcome was the cumulative count of confirmed COVID-19 from March 21 to March 27, 2020 from the Center for Systems Science and Engineering at Johns Hopkins University, Baltimore.

Additional covariates included altitude, GDP per capita, health expenditure as a percent of GDP, life expectancy, percentage of inhabitants aged 65 years or older, the Infectious Disease Vulnerability Index, urban population density, number of flight passengers per capita and closest distance to a country with an already established epidemic.

Study reliability

The evidence has several shortcomings that prevent firm conclusions to be drawn, these include: considerable differences in testing practices between areas meant the estimates of  COVID-19 were not reliable. Only 38 areas had implemented public health interventions by the cut-off date of March 11, 2020. There was variation in social distancing measures across areas. The mass gatherings were analysed irrespective of the size of mass gatherings. The research was unable to quantify compliance with social distancing and restrictions of mass gatherings. Temperature and humidity were collected for the capital of each geopolitical area, which may not have accurately represented area-wide climate patterns. Finally, the time window of one week will not adequately assess variations in temperature and humidity.

Clearly defined setting Demographic characteristics described Follow-up length was sufficient Transmission outcomes assessed Main biases are taken into consideration
Unclear * No No Unclear ** No
* Diagnostic test data was not reliable in many settings
** Diagnostic test data was not reliable in many settings

What else should I consider?

Environmental studies require very accurate data to draw firm conclusions. Temperature and humidity data cannot simply be extracted for the capital and then applied to the whole country. Similarly, public health measures such as mass gatherings need to be extracted in detail. The impact of climate needs to be studied over a longer period of times to assess associations. It is difficult to determine which component is contributing to the outcome when a number of interventions and exposures are studied over a short window of time.

About the authors

Carl Heneghan

Carl is Professor of EBM & Director of CEBM at the University of Oxford. He is also a GP and tweets @carlheneghan. He has an active interest in discovering the truth behind health research findings

Tom Jefferson

Tom Jefferson, epidemiologist.