Study: COVID-19 May Be a Seasonal Virus

Key Takeaways

According to a new study, SARS-CoV-2 is largely seasonal, meaning that cases are highest during the colder months of the year (like influenza).
The findings could explain why some countries and regions have been hit harder by the pandemic than others. The study could also help researchers better model viral epidemiology.
Seasonality is not the only factor that affects viral prevalence and virulence—public health policy and social attitudes also contribute.

Researchers at the University of Illinois at Urbana-Champaign have found that COVID-19 incidence and mortality rates are strongly correlated with temperature and latitude—a measurement of distance north or south of the equator.

The researchers relied on the website Worldometer and other sources to help them determine whether the infectious characteristics of SARS-CoV-2, the virus that causes COVID-19, (including incidence, mortality, recovery cases, active cases, testing rate, and hospitalization) varied between countries.

They specifically looked at three factors:

Average spring temperature
Latitude (distance north or south of the equator)
Longitude (distance east or west of the prime meridian)

The researchers began their investigation by pulling relevant data for the day of April 15 from 221 countries around the world. That date is significant because it is the one moment in a year where seasonal temperature variation is at its maximum worldwide.

The statistical analysis confirmed what the researchers had suspected from the start: COVID-19 incidence, mortality, recovery cases, and active cases decreased as temperature increased, decreased as latitude decreased, and were not affected by longitude.

Researchers found warmer countries that are closer to the equator have had fewer COVID-19 cases and deaths than colder countries that are farther away from the equator. The study was published in the journal Evolutionary Bioinformatics in January.

What This Means For You

While more research is needed, a new study has shown that the COVID-19 virus is likely to be seasonal. Other viruses, like those that cause influenza, are also seasonal. In the U.S., these viruses tend to get worse in the winter months.

What Is Causing Seasonality?

Many viruses experience climate-associated fluctuations in prevalence and virulence. The influenza A and B viruses wax and wane with the seasons; in fact, when we talk about the flu it’s often preceded by “seasonal.”

Viral activity typically peaks in the fall and winter—when the weather is colder—and plummets in the spring and summer—when the weather is warmer.

Gustavo Caetano-Anollés, PhD, senior study author and professor of bioinformatics in the department of crop sciences at the University of Illinois at Urbana-Champaign, tells Verywell that scientists are “still struggling” to explain the phenomenon, but there are several hypotheses.

Environmental Factors

“Environmental factors (temperature, humidity, UV levels, surface interactions, etc.) could directly affect the viability of the virus, decreasing virus numbers and the chances of viruses causing disease,” Caetano-Anollés says. For example, intense or prolonged exposure to radiation from the sun can inactivate viruses.

Human Biology and Activities

The increased incidence of disease in the winter could also have something to do with how the cold and darkness in our environment changes our biology and behavior.

Caetano-Anollés says that low ambient temperatures and nutritional deficiencies can be detrimental to “virus-specific immunological responses.” Another theory is that the depletion of our vitamin D stores can indirectly render us more susceptible to a flu virus.

What Is Vitamin D Deficiency?

Stanley Perlman, MD, PhD, professor of microbiology and immunology at the University of Iowa’s Carver College of Medicine who was not involved in the study, tells Verywell that engaging in “indoor activities with more people in close quarters” also “contributes to person-to-person spread.”

When we spend extended time with other people indoors because it’s cold outside, that, in turn, can contribute to the increased spread of the viruses during the winter months.

Viral Evolution

The third possibility is that viruses evolve in the winter to become more virulent. In this paradigm, Caetano-Anollés says that “genomic changes in the viruses [are] the ones responsible for seasonality.”

However, Caetano-Anollés’ research indicates that these factors are not to blame in the case of COVID-19.

Will COVID-19 Get Worse In the Winter?

Why Seasonality Matters

Caetano-Anollés says that the results might partly explain why some countries have been more successful at containing the spread than others. For example, despite having more than one billion people, Africa has reported only 3.5 million cases and 88,993 deaths as of January 28, 2021—far better numbers than has been seen in North America and Europe.

However, Caetano-Anollés says that temperature and latitude are not the only variables of consequence. Public health policies and cultural differences in social cohesion likely play a role in the specific outcomes for continents, regions, and countries.

It’s All Relative

“Note that everything is relative,” Caetano-Anollés says. “If you have a raging pandemic (e.g. Brazil), the seasonality phenomenon will only mitigate a fraction of the cases.”

Under the leadership of President Jair Bolsonaro—who has been vocal about his belief that COVID-19 is nothing more than a “measly cold”—Brazil has emerged as a world leader in COVID-19 infections. At one point over the summer, the country had a daily death toll on par with that of India and the U.S.

Gustavo Caetano-Anollés, PhD

The big distinction [between SARS-CoV-2 and] the yearly flu is that we are facing a global pandemic of unprecedented proportions.

— Gustavo Caetano-Anollés, PhD

“I think that seasonal factors are just one part of the total picture,” Perlman says. “As we learned last summer when SARS-CoV-2 peaked in some places in the U.S., even though temperatures were high.”

The researchers noted in their study that the correlations between temperature and latitude and COVID-19 severity did not hold true for the internal U.S. They also did not find any correlation between temperature, latitude, and longitude, and rate of SARS-CoV-2 viral evolution, which disproves the third theory of viral seasonality.

Caetano-Anollés stated in the study’s press release that the findings suggest that “seasonal effects are independent of the genetic makeup of the virus.”

Will the Study Change Our Understanding of the Virus?

The study’s findings suggest that SARS-CoV-2 behaves somewhat like the flu because it appears to be more severe in the colder months of the year and milder in the warmer months.

However, Caetano-Anollés cautions against taking the analogy too far: SARS-CoV-2 is an entirely different beast and should be treated as such. “The big distinction [between SARS-CoV-2 and] the yearly flu is that we are facing a global pandemic of unprecedented proportions,” he says.

Coronavirus (COVID-19) and the Flu: Similarities and Differences

Caetano-Anollés says that under these circumstances, seasonality will “manifest as waves, without any period with very few infections (such as in the case of the flu).” He expects incidence rates to dip once a critical number of vaccines have been administered.

The results of the study do have implications for ongoing research as well as the development of treatments. The revelation that SARS-CoV-2 is seasonal could potentially improve the efficacy of prevention initiatives. Caetano-Anollés points out that seasonality “permits better mitigation and allows time for vaccine deployment, as we do every year with the flu.”

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.