Michigan’s annual flu season stretches throughout most of the cold months of the year, with the peak typically occurring between December and February.
As the state wraps up its second summer season with coronavirus active in the community, and heads into its second fall and winter with the virus, it raises the question — will COVID become the next seasonal illness?
Seasonal viruses are those that tend to circulate and peak during specific seasons of the year. In temperate climates like Michigan, viral infections like influenza typically increase during the colder months due to a variety of factors, including increased crowding of people indoors, a reduction in vitamin D levels without sufficient sunlight, and the suppression of immune systems under colder conditions.
While behavioral factors during the colder months produce ideal conditions for viral transmission, doctors say it’s too early to predict future seasonal patterns with the coronavirus.
“Anyone who says this is a seasonal virus right now, I think they’re getting ahead of themselves,” said Dr. Liam Sullivan, an infectious disease specialist for Spectrum Health in Grand Rapids.
“This virus has not shown any seasonality to it to this point in time, and I think that’s for a couple reasons.”
For starters, Sullivan said, the vast majority of people on the planet still don’t have a level of immunity against the virus, either by vaccination or natural infection. While about 70% of the U.S. population has gotten a first dose, other countries have much lower rates, many due to lower vaccine access.
The virus has so many vulnerable targets to go after year-round, that it doesn’t require ideal conditions to spread. That’s evident by the significant outbreaks throughout all four seasons and across both the northern and southern hemispheres.
Another factor is the continued mutation of the coronavirus into more infectious strains like the delta variant, which allows it to continue spreading at a high rate year-round.
“This virus is kind of just writing its own script and doing things it wants to as it goes along,” Sullivan said. “There are things we know that increase the propensity of its spread but there’s still some things that we don’t quite know about its behavior yet.”
“Is it possible that it could turn into a seasonal virus? Sure it is. But I think it’s too early to say that and I don’t think there’s been any seasonal pattern to this virus yet.”
Dr. Christopher Ledtke, an infectious disease specialist for Munson Healthcare, echoed Sullivan. Instead, he said, coronavirus is proving to be more “surge-based.”
While Michigan saw a significant rise in new COVID cases in the spring of 2021, even as temperatures were warming in the Midwest, other parts of the country were reporting plateaus and declines in new cases.
Fast forward to the summer, and the south has been the leading region for new COVID cases since early July, despite weather conditions not being ideal for transmission of other seasonal viruses.
“You have local surges in specific parts of the country and different parts of the world, on the north side of the equator, south side of the equator, completely unrelated to what season is happening,” Ledke said. “I would not think of this as a seasonal virus in any capacity. It’s based on local surges, lightly related to new variants being introduced into the area.”
That doesn’t mean there aren’t factors associated with certain seasons that will further the spread of coronavirus. For example, Sullivan said it would be naïve to ignore that schools play a significant role in increased transmission during the non-summer months. The same can be said about influenza numbers annually.
“The major theme with this time of the year is just people are together more and they’re closer together and they’re in potential settings where the virus can more easily spread from person to person,” Sullivan said.
Dr. Laraine Washer, a clinical professor of infectious disease within the University of Michigan Medical School and epidemiologist for Michigan Medicine, said “we do not have enough longitudinal experience with COVID to firmly predict seasonality for the coming year or two.”
Washer does expect that COVID will eventually evolve into an “endemic seasonal respiratory virus,” meaning it will be a consistently present virus circulating in pockets of the global populations.
That thinking is on par with 89% of the more than 100 immunologists, infectious disease researchers and virologists surveyed by the international journal Nature. About 60% said SARS-CoV-2 was very likely to become an endemic virus, compared to 29% who said it was likely, 5% said it was unlikely, and 6% said there wasn’t enough evidence to estimate.
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