Covid-19 transmission may have seasonal spikes tied to temperature and humidity, increasing at different times of the year for different locations, a new study suggests.

Colder regions, such as the U.S. Northeast, may experience more cases during winter, while warmer regions, such as the southern United States, may see higher transmissions in the summer. More temperate zones could experience two seasonal peaks.

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“We think covid may be becoming endemic, meaning that it will stay within the human population, but we will see spikes based on where we are in a particular geographic point,” said author Antar Jutla, a hydrologist at the University of Florida. “The severity of peaks will be defined by how colder the temperature is and how warmer the temperature is.”

Jutla and his colleagues found that covid cases in 19 hot spots worldwide increased above and below certain temperature and humidity thresholds, due to the virus’s movement as an aerosol and human behavior. Cases spiked when air temperatures dipped below 62 degrees (17 Celsius) or above 75 degrees (24 Celsius). The virus also tends to linger more in drier environments compared to humid ones.

We need to basically design the intervention or mitigation strategies based on the environment in which we live,” said Jutla. “Regions like Florida, India, Africa – they are warm regions…. They basically get hit by these waves, but at different times than what happens in the northern parts, the colder regions.”

While cases could pop during extreme chills or heat waves, Jutla said such temperatures typically need to persist for about a 14-day average. The team validated the data to December 2021, but characteristics could change with new mutations or new variants.

Aerosol researcher and co-author Chang-Yu Wu explained that local humidity and temperature play vital roles in the size of the virus’s particles, which can influence its life span in the air. Drier atmospheres in colder regions will induce water evaporation from the particles, shrinking their size and allowing them to float in the air for longer periods. People also tend to seek shelter inside in colder environments and expose themselves to recirculated air that potentially contains the virus.

Humid, hotter environments contain more water content in the air, which can condense onto the virus particles, make them bigger and theoretically fall to the ground faster. Wu compares the particles to a rock in this case – the more mass, the faster it falls. At the same time, people often move inside to avoid the warmth and again expose themselves to recirculated air. Air conditioners also dry the inside air, creating an environment similar to that of winter for aerosols.

“Human behavior 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.”

Fewer cases were detected between 62 to 75 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.

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

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