Will COVID-19 become a seasonal disease like the flu? It's too early to tell, experts say

As governments around the world try to turn the tide on the spread of COVID-19, doctors are wondering if the disease will truly be eradicated or if we'll see it become a seasonal infection like the flu and common cold.

Why certain diseases and infections have seasons isn't completely understood because a variety of factors come into play. 

But epidemiologists — those who study diseases — are waiting to see if these factors may influence COVID-19, making it a seasonal disease.

"I think it's probably a little bit too early to tell," said Selena Sagan, an associate professor in the department of microbiology and immunology at McGill University in Montreal. "We really don't know enough about this virus yet. And the fact that it's really a global pandemic suggests that transmission is readily occurring right now."

Every disease has a different pattern.

Influenza seems to thrive in cooler, drier weather. The measles and chickenpox — in the U.S. and Canada — emerge during the school calendar year. 

A 2018 study by Micaela Martinez, an infectious disease ecologist at Columbia University's Mailman School of Public Health, found that cases of Middle East respiratory syndrome (MERS), a dangerous coronavirus linked to camels, are reported more often during calving season, though it's not understood exactly why.

The study even found that infections and diseases one might not readily associate with ebbs and flows — such as gonorrhea and genital herpes — also are linked to seasons.

"Our planet is seasonally structured, no matter where you live," said Martinez. "Being in temperate regions like in Canada or the U.S. where we're far away from the equator, we have the four seasons. If you're in an equatorial place, it's still seasonal, it's just not the four seasons. It's the rainy and dry season."

In the northern hemisphere, it might be the cold that influences the spread of an infection such as the flu. But in equatorial regions, hydrological conditions and humidity would allow mosquitoes to breed and then spread diseases like malaria. 

And not all the seasons that seem linked to particular infections have to do with humans. Many of them are linked to "vectors," the carrier of a disease such as mosquitoes or bats or even agricultural seasons.

Humidity matters

Epidemiologists believe that humidity in particular may be responsible for playing a role in infections in various ways, even in the northern hemisphere, where the flu is widespread.

In fact, evidence has shown that higher humidity can lower the spread of the flu.

"We do know that ambient conditions — and it seems to be particularly humidity — modulate the survival of the influenza virus," said Dr. Jeffrey Shaman, a professor at Columbia and director of its climate and health program. "Now why that is, we don't know. But it's something that's been observed over and over again."

Social patterns

With influenza (the flu), for example, the high season comes at a time when colder, darker days dominate.

Epidemiologists believe that part of the increased spread in winter is because of the social habits we form at that time of year. 

"When it's colder, people tend to spend more time indoors, in close proximity to one another," said Shaman. "And that contact with one another might facilitate the transmission of influenza and then, therefore, we see a spike of flu in the wintertime."

Additionally, some hypothesize that it could also be something to do with our immune system during fall and winter.

"When you're in temperate regions and more polar regions, you don't have as much sunlight exposure during the wintertime," Shaman said.

"The sun's at a lower angle in the sky and the day is shorter, and as a consequence, your melatonin and vitamin D production is dropped. That can suppress immune function, which might mean that not only are you perhaps more susceptible to infection, but when you are infected, you're going to have a more severe infection, and you're going to shed more of the virus, so you're going to be more contagious." 

Wait-and-see with COVID-19

But do any of these variables matter when it comes to COVID-19? 

Many diseases and infections, such as the flu and malaria, have been around for decades, so more people have developed an immunity to them. That's because more people have been exposed.

But COVID-19, and the virus that causes it, SARS-CoV-2, have only been around for approximately four months. It will take some time for the wider population to develop some sort of immunity. At present, most people are susceptible.

It could take several months to more than a year to determine if the disease will be seasonal or not.

"I find that the conversation about SARS-CoV-2 going away in the summer, because of the seasonality, as really being quite a false hope," said Martinez. "That's all based on what we've seen for other pathogens, where there's immunity, and right now we just don't have that in the population."

Martinez said that while cases might drop when warmer weather arrives in the northern hemisphere, it doesn't mean COVID-19 will disappear. Even the flu stays around during the summer; it's just not as prevalent.

"These things are incredibly complex," Martinez said. "I've had 100 years of polio data for the U.S. and worked tirelessly for five years or six years on it, and I still can't give you an exact answer [about it]. And so that's why I say that when people are putting these ideas [about COVID-19 potentially being seasonal] out there, it's just too premature."