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Warmer weather by itself won’t curtail the spread of covid-19, expert panel finds

Human behavior plays a bigger role in determining the course of the coronavirus pandemic than any weather or climate factors, their report finds.

People spend spring break at the beach in Miami Beach on Monday. (Cristobal Herrera-Ulashkevich/EPA-EFE/Shutterstock)

Early on in the coronavirus pandemic, when our knowledge of the virus was limited, it appeared there might be connections between a country’s weather and climate and virus transmission. A slew of studies, most of them released before peer review, explored such potential linkages.

While some of the studies came to contradictory conclusions, a narrative emerged that warmer countries tended to see less severe outbreaks, including those in Southeast Asia, such as Thailand. But as scientists involved with a World Meteorological Organization-led (WMO) effort studied the relationship between the deadly pandemic, meteorological factors, and air quality, they’ve come to see that things like mask mandates and other government interventions have had a far bigger influence than weather on the course of the virus.

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Transmission dynamics, and the timing of virus surges, have been driven more by changes in human behavior, demographics, and lately, more transmissible variants than by changes in the weather, a country’s climate, or air quality, according to an initial report by a WMO task team report released Thursday.

“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,” Ben Zaitchik, a co-chair of the task team who is an Earth scientist at Johns Hopkins University, said in a statement. “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 16 interdisciplinary experts brought together under the auspices of the WMO, which is a United Nations agency, assessed peer-reviewed studies published through the first week of January on the relationship between factors such as air quality, temperatures, humidity, and precipitation and the transmissibility of the virus.

The experts’ message provides cold comfort, since it shows that the arrival of spring and summer in the Northern Hemisphere won’t be sufficient by itself to curtail new cases or prevent another surge in infections. In the U.S., cases have declined considerably from a peak seven-day average of about 248,209 on Jan. 12, to about 55,000 new cases per day as of this week. However, the number of cases has flatlined and even increased in parts of the U.S., and Europe has seen a recent surge, despite the arrival of milder weather there.

In addition, Brazil, despite its mild climate featuring much of the Amazon region, has been suffering from a deadly spike in covid-19 cases, with a particularly contagious and deadly variant emerging there.

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The hope has been that spring and summer could alter human behavior by encouraging outdoor social activities in which it is harder to transmit the virus — affording the U.S. an opportunity to gird against a fall and winter surge through extensive vaccination campaigns. However, more transmissible and possibly deadlier variants, plus vaccine hesitancy, could thwart such progress, Zaitchik said. In addition, despite showing sensitivity to ultraviolet light and greater humidity in lab settings, the virus has proven robust even in the summer.

While the experts cite evidence showing it’s easier to transmit the virus via the air within confined spaces than it is outdoors, they cautioned that when it comes to direct environmental effects on virus survival, “no firm conclusions can be drawn for COVID-19 at this time.”

“It’s clearly able to transmit across climate zones and across seasons,” Zaitchik said in an interview.

Rachel Lowe, a report co-author and researcher at the London School of Hygiene and Tropical Medicine, said viral transmission is currently most controlled by government interventions (such as lockdowns and business closures) and the susceptibility of populations.

“We’re in a stage of the pandemic where the impact of weather would be secondary,” she said in an interview. “Gathering outdoors is definitely preferable we still have to be extremely careful and not let down our guard.”

The report incorporates research published by the first week of January, so it does not include much of the new information on infections caused by the new variants, which have become the dominant strain of the virus in countries in Europe and which are increasing rapidly in the U.S.

The coronavirus is deadly enough. But some experts suspect bad air makes it worse.

Given the seasonality of other respiratory infections, such as the flu, there has been an assumption shared by many in the medical community that covid-19 might gradually evolve into a seasonal disease, such as the flu. However, the new report finds that direct weather influences on the virus don’t clearly influence transmission rates, at least not yet. This is consistent, Zaitchik said, with a new virus encountering a population that has never been exposed to it before.

The report also finds that, while there is convincing evidence that poor air quality increases mortality from covid-19, pollution may not directly affect the transmission of the virus itself.

The task team is going to continue to study the ties between weather, climate, air quality and covid, especially now that the initial flurry of papers is over with. “We’ve learned a lot about how to do this research,” Zaitchik said. “We’ve caught our breath and said, ‘okay, let’s do this right.’”