Since the beginning of the pandemic, scientists and the public have speculated that, as with cold-causing viruses, meteorological factors like warm or cold weather may influence the spread of COVID-19.

Now a new report cautions that weather and climate conditions, including the onset of higher temperatures during spring, should not be used as a trigger to relax COVID-19 transmission reduction measures. The report is the first from a World Meteorological Organization COVID-19 Task Team—composed of 16 experts from NOAA’s Climate Program Office and other meteorological services around the world—assessing whether meteorological and air quality factors affect the pandemic.

Government interventions, such as mask mandates and travel restrictions, rather than meteorological factors appear to have primarily influenced COVID-19’s spread in 2020 and early 2021, according to the Task Team. Other relevant drivers include changes in human behavior and demographics of affected populations, and more recently, virus mutations. 

“At this stage, evidence does not support the use of meteorological and air quality factors as a basis for governments to relax their interventions aimed at reducing transmission,” said Task Team Co-chair, Dr. Ben Zaitchik, Department of Earth & Planetary Sciences, Johns Hopkins University. “We saw waves of infection rise in warm seasons and warm regions in the first year of the pandemic, and there is no evidence that this couldn’t happen again in the coming year.”

The report summarizes key findings published by the first week of January 2021. It therefore does not include peer-reviewed literature regarding weather, climate, and air quality’s influence on the spread of the new COVID-19 virus strains, or on the severity of infections caused by these new strains.

“This report is a great synthesis of what we know so far, but it also highlights that we still don't know enough about the seasonality of this disease over the long term,” said Juli Trtanj, One Health and Integrated Climate Research Lead with NOAA’s Climate Program Office. “NOAA and our health partners will continue to investigate the role of climate and weather factors so that we can produce actionable information to reduce risk and support the pandemic response.”

Role of seasons and air quality unclear

Respiratory viral infections frequently show some change with the seasons, in particular a peak during fall and winter for influenza and cold-causing coronaviruses in temperate climates. This has fueled expectations that, if it persists for many years, COVID-19 will prove to be a strongly seasonal disease. 

However, “the underlying mechanisms that drive seasonality of respiratory viral infections are not yet well understood,” says the report’s executive summary. “A combination of direct impacts on virus survival, impacts on human resistance to infection, and indirect influence of weather and season via changes in human behavior may be at work.”

“Laboratory studies of SARS-CoV-2, the virus that causes COVID-19, have yielded some evidence that the virus survives longer under cold, dry, and low ultraviolet radiation conditions. However, these studies have not yet indicated if direct meteorological influences on the virus have a meaningful influence on transmission rates under real world conditions,” according to the executive summary. 

The influence of air quality factors is still inconclusive. There is some preliminary evidence that poor air quality increases COVID-19 mortality rates, but not that pollution directly impacts airborne transmission of the virus that causes COVID-19, according to the Task Team.

The report focuses on outdoor meteorology and air quality conditions and does not address details of indoor air circulation.

COVID-19 Task Team to update scientific evidence, advise on good modeling practices

WMO’s Research Board established the interdisciplinary and international Task Team to rapidly summarize the state of knowledge regarding potential meteorological and air quality influences on COVID-19 dynamics, given the staggering number of papers and pre-prints currently available. 

“The rapid pace of COVID-19 research has meant that studies with limited data appeared faster than the information could be cross-checked and peer-reviewed. It soon became clear that reported evidence was often contradictory or selective due to methodological and data-related shortcomings. The WMO Task Team therefore seeks to encourage good practice in research and communications,” says Prof. Juerg Luterbacher, Director, Science & Innovation, and WMO Chief Scientist.

Future work will include updating the scientific evidence over the next months, as well as identifying and promoting a structured set of priority research questions, objectives and priorities for research investment within the pandemics – weather – climate-air quality nexus. 

The Task Team will also advise and inform on good practices and minimum standards for integrated infectious disease modelling methods considering environmental determinants, and recommend on how the coronavirus, climate, weather and air quality nexus should be factored into research and information delivery in future WMO activities.

Read the report »

This story was adapted from a WMO press release.