Expert Commentary

COVID-19 infections rise in cold temperatures year-round, study confirms

Prior research on weather and COVID-19 transmission provides inconsistent results. A recent study shows transmission rates rise when temperatures drop or outdoor levels of ultraviolet radiation are high.

covid infection transmission winter cold weather

As temperatures drop and winter conditions intensify, COVID-19 infection rates easily could rise, warns statistician Ran Xu, an assistant professor at the University of Connecticut’s Department of Allied Health Sciences.

A recent study he co-authored confirms a link between weather and coronavirus transmission, which, regardless of where you live on the planet, tends to slow in hot weather and increase when it’s cold. Prior studies had provided inconsistent results. Researchers still aren’t sure what the main driver behind the change is, however, Xu notes.

He says there is biological evidence the virus is more active at lower temperatures. But people also behave differently across seasons. When it’s chilly and snowy, they’re more likely to gather indoors, where there’s less air circulation, he adds. On warm, sunny days, people tend to spend more time outside and keep doors and windows open at home.

Xu and his fellow researchers discovered a correlation between temperatures above 25 degrees Celsius, or 77 degrees Fahrenheit, and lower rates of COVID-19 infection. They also learned that people are at a higher risk of catching COVID-19 when temperatures are low and outdoor levels of ultraviolet radiation are very high.

Their paper, “Weather, air pollution, and SARS-CoV-2 transmission: a global analysis,” appears in the October 2021 edition of The Lancet Planetary Health. Xu and Hazhir Rahmandad, an associate professor of system dynamics at MIT’s Sloan School of Management, are the lead authors.

The study differs from earlier research on weather and COVID-19. The main difference is that the current study accounts for the delay between when someone contracts COVID-19 and when public health officials record their positive test result.

“We are trying to associate weather with the time when people get exposed to the virus,” Xu explains, adding that the delay lasts an estimated 10 to 14 days. “A lot can change [over that period].”

Prior studies focus on what the weather was like on the date a positive COVID-19 case was recorded. But that data is an “inappropriate” measure to use, according to a review of the academic literature published earlier this year on the topic, “Data-related and methodological obstacles to determining associations between temperature and COVID-19 transmission.”

That analysis, which appears in Environmental Research Letters, also finds that most of the peer-reviewed studies evaluated “were rushed to completion” and have substantial,” flaws, including in their methodology or data design.

“It is our viewpoint that most studies of the identified 70 publications have had significant flaws that have prevented them from providing a robust scientific basis for the association between temperature and COVID-19,” write the authors, led by Zhaomin Dong, a researcher at Beihang University’s School of Space and Environment.

Study details

The study in the Lancet Planetary Health examines various data collected from 3,739 locations worldwide between Dec. 12, 2019 and April 22, 2020. The authors looked for relationships between COVID-19 infection rates and environmental variables such as wind speed, temperature, precipitation and ultraviolet, or UV, radiation. They also looked for links between transmission and air pollutants, including ozone.

The study’s findings “suggest that many temperate zones with high population density could face larger risks in winter, whereas some warmer areas of the world might have slower transmission rates in general.”

The study also reveals an unexpected association between COVID-19 infection and the strength of outdoor levels of UV radiation. The researchers write they had predicted high levels of UV exposure would reduce transmission “due to both stimulating vitamin D production and ultraviolet’s disinfecting effects.”

Their analysis instead shows COVID-19 transmission drops when levels of UV radiation are moderate, but increases when UV levels are very high. They reason that the “unexpected increase in the high ranges of ultraviolet could be due to a shift of social interactions into higher risk, indoor, settings when ultraviolet amounts are very high.”

The findings, Xu and his colleagues write, “suggest year-long changes in transmission rates due to weather might partly explain some waves of the pandemic across different nations and substantially moderate the risk profile over the coming year. Nevertheless, as shown in 2020, the estimated effect of weather on transmission risk is not large enough in most places to quell the epidemic alone.”

Reporting on COVID-19 infection: Guidance for journalists

Xu and two of his co-authors — Rahmandad and Navid Ghaffarzadegan, an associate professor in the Grado Department of Industrial and Systems Engineering at Virginia Tech — offered advice for journalists covering the coronavirus during the winter months.

It’s crucial, they say, that journalists remind audiences that individuals can help control the spread of COVID-19 by changing and monitoring their own behavior. Wearing masks, avoiding crowds, employing social distancing and getting vaccinated are even more important in the winter.

While researchers figure out what exactly causes transmission rates to rise when it’s cold, news reports should explain the consequences of spending a lot of time indoors.

“We don’t know the mechanisms [behind higher COVID-19 transmission rates], but the chances are that they are more behavioral than biological: what drives people indoors increases transmission,” Rahmandad wrote via email.

Journalists also must keep in mind that various other factors — for example, population density — play a role. COVID-19 transmission probably will be higher in areas where more people live, Ghaffarzadegan wrote in response to questions from The Journalist’s Resource.

“Journalists should know that when we are talking about the effect of temperature on transmission, we are talking about multiple mechanisms, including the infectivity of the virus, human body response, as well as social and behavioral reactions to change in weather,” he explained.

COVID-19 vaccination rates, which vary widely across the planet, complicate the issue.

In the U.S., 59 people out of every 100 people in the population are fully vaccinated, meaning they have received both doses required under a two-dose regime, according to the World Health Organization. In comparison, less than 1 in every 100 people in Haiti, Yemen and Chad are fully vaccinated.

In Chile, Cuba, Denmark, Singapore and a handful of other countries, more than 80 people out of 100 are fully vaccinated, WHO data show.

Journalists “should consider that, now, the vaccination rate plays a huge role in the number of cases and death rate and it is not easy to compare different regions of different temperatures,” Ghaffarzadegan wrote. “The main message in simple words is that, if everything else is constant, we are more vulnerable to COVID in colder temperatures so we should be ready and prepared.”

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