“Human behaviour is a very important factor in the transmission,” said Wu, who is also a professor at the University of Florida. “It’s not just purely the physics or biology [of the virus] that dictates.... It’s a combination of these.”

Loading

Fewer cases were detected between 17 to 24 degrees, considered more tolerable temperatures for people to be outside.

“This study confirms the previous findings of seasonality with SARS-CoV-2 infection (low temperature and humidity),” Mohammad Sajadi, a professor at the Institute of Human Virology at the University of Maryland School of Medicine, wrote in an email. “It also adds to our knowledge about transmission in warmer temperatures.” Sajadi was not involved in the study.

He said more work is needed to further explore the use of indoor air conditioning, stating that “the United States and India likely have different levels of usage, which was not accounted for in this study.”

“If anything, [the study] again shows us how complicated the issue of seasonality is,” Sajadi wrote.

Sajadi compared the findings to the seasonality, or climatic conditions, of influenza as well. For influenza infections, he said the stability of aerosols also occurs at two vastly different levels of humidity. The different patterns of seasonality depend on the geographic locale, too.

Loading

Others found the climate-COVID relationships in different locations plausible as well.

“I like that the study points out that climate-COVID relationships will differ by region,” emailed Ben Zaitchik, co-chair of a COVID-19 research task team at World Meteorological Organisation and not involved in the study. “We found something similar in a study . . . and I think it’s important to emphasise.”

He also stressed that linking climate to COVID risk is an evolving field of research. Several studies with other climate-COVID mechanisms have been published in the past few months, including one discussing the challenges of studying such an area.

It is important to remember that these are only one driver of risk, wrote Zaitchik. “All evidence is that it has been a secondary driver up to this point in the pandemic. It might become more important as COVID becomes endemic and settles into a seasonally locked pattern - which many expect that it will.”

Considering the virus’s environmental and socioeconomic influences, Jutla and colleagues also created a model to predict the risk of COVID in the United States weekly.

The model, currently in the beta phase, includes air temperature, humidity, population density, economic stability (income), age, diversity, housing, vaccination data sets and more. The predictive model builds on work by Jutla and other co-authors, who previously demonstrated how cholera outbreaks are tied to environmental factors such as air temperature and salinity.

“Social behavioral sciences need to be strongly intertwined with the natural sciences,” said co-author Rita Colwell, a biologist at the University of Maryland who helped uncover the role the environment plays in cholera transmission. “The intersection of the physical environment and human behaviour in warm and cold regions is exemplified in the COVID case reports.”

The model runs around 80 to 90 per cent accuracy nationwide (validated on a US county-level scale), although Zaitchik said it is important to see the model in a peer-reviewed form especially because of the model’s novelty. “There hasn’t been any robust demonstration of a model like that for COVID-19 risk prediction,” he said.

Overall, the researchers stressed that proper ventilation and filtration, such as masks, are among the most important measures for preventing transmission of the disease. Air sampling studies have repeatedly shown that excellent ventilation and filtration can reduce COVID case loads, even in larger, relatively crowded places such as gyms.

“For me, the biggest takeaway is the need to focus on the built environment,” Zaitchik wrote. “It’s not simply a matter of ‘is it cold out’; it’s a matter of whether those cold temperatures are leading you to crowd indoors, reduce ventilation, use climate controls that result in low indoor humidity, etc.”

Washington Post