Novel coronavirus really is seasonal, study suggests

Warm temperatures and tropical climates may really help reduce the spread of COVID-9, a new study suggests.

The study found that places with warm temperatures and long hours of sunlight — such as countries close to the equator and those experiencing summer — had a lower rate of COVID-19 cases, compared with countries farther away from the equator and those experiencing colder weather.

The findings held even after the researchers took into account other factors that could affect both the spread of COVID-19 and the number of reported cases, such as a country's level of urbanization and the intensity of COVID-19 testing.

Still, the authors stress that their findings don't mean that summer weather will eliminate COVID-19; but it may give people a leg up against the disease.

"Our results do not imply that the disease will vanish during summer or will not affect countries close to the equator," the authors wrote in their paper, published April 27 in the journal Scientific Reports. "Rather, the higher temperatures and more intense UV [ultraviolet] radiation in summer are likely to support public health measures to contain SARS-CoV-2," the novel coronavirus causing COVID-19.

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Seasonal virus 

Shortly after the COVID-19 pandemic began in the winter of 2020, there was speculation that summer temperatures may bring relief from COVID-19. Indeed, many respiratory viruses, including flu viruses, show a seasonal pattern, peaking during the winter and dipping during the summer.

Scientists don't know for sure why these viruses follow a seasonal pattern, but a number of factors are thought to play a role. For example, studies suggest that many respiratory viruses are more stable and linger in the air longer in environments with cold temperatures and low humidity, Live Science previously reported. Human behaviors, such as gathering indoors in wintertime, could also boost transmission.

Studies in lab dishes have also found that high temperature and humidity reduce the survival of SARS-CoV-2, but whether this translates to real-world transmission was unclear.

In the new study, the researchers analyzed information from 117 countries, using data on the spread of COVID-19 from the beginning of the pandemic to Jan. 9, 2021. They used statistical methods to examine the relationship between a country's latitude — which affects the amount of sunlight it receives as well as temperature and humidity — and its level of COVID-19 spread. They also used data from the World Health Organization to control for factors that could affect how hard a country is hit by COVID-19, such as air travel, health care expenditure, the ratio of older adults to younger people and economic development.

They found that every 1 degree increase in a country's latitude from the equator was tied to a 4.3% increase in the number of COVID-19 cases per million people. This means that if one country is 620 miles (1,000 kilometers) closer to the equator compared with another, the country closer to the equator could expect to have 33% fewer COVID-19 cases per million people, with all other factors being equal between the countries.

"Our results are consistent with the hypothesis that heat and sunlight reduce the spread of 

SARS-CoV-2 and the prevalence of COVID-19," according to the authors, from the Heidelberg Institute of Global Health in Germany and the Chinese Academy of Medical Sciences in Beijing. The findings also mean that "the threat of epidemic resurgence may increase during winter," as was seen in many countries in the Northern Hemisphere in December 2020 and January 2021, they said.

The authors note that their study only included data up until Jan. 9, 2021, before a number of COVID-19 variants, including variants that first emerged in South Africa and the U.K., took off around the world, so it's unclear whether these variants will show similar patterns of seasonal infection.

Originally published on Live Science.