| Since the pandemic's early days, people have been waiting for Covid to behave in a way that feels familiar — to eventually fade from our daily existence and behave more like, say, influenza, which only bothers us once a year. But years after the start of the pandemic, Covid cases are still present year-round. Experts agree it cannot be considered seasonal – at least not yet. This begs the question, what makes a virus seasonal? Like so many things Covid, this, too, doesn't exactly have a straightforward answer. When I asked Vanderbilt infectious disease specialist William Schaffner to define seasonality, he actually laughed. "If I could give you a definitive answer to that, somebody would put me on the list as a possible Nobel Laureate," he said. "It is remarkable, still in 2023, how little we can understand these seasonal behaviors." Seasonality is determined by patterns in case data. Flu cases, for instance, start to spike in late October in the Northern Hemisphere, reach a peak at the end of December and fall to nearly zero by late February. By contrast, using hospitalizations as a proxy for cases, Covid appears to have some dips throughout the year, but also surges in both the winter and summer. As for what actually causes a virus to fall into a seasonal pattern, the answer ranges from environmental factors to human behavior. Respiratory illnesses like the flu spike in the winter because the dry air allows the virus to linger in the air for a longer time, Schaffner says. On top of that, people congregate indoors in the winter for holiday gatherings and to escape the cold, increasing exposure. Although it is also a respiratory virus, scientists still don't know all the reasons why Covid can thrive in the summer as well as winter. At least some of it may have to do with human behavior. This year, for example, experts suggest that extreme heat waves driving people inside along with summer travel may have led to Covid surges in July and August. Because Covid is relatively new to the human population, it may just take more time for it to fall into a seasonal pattern. And that pattern matters. It helps public health better plan for and respond to outbreaks. If Covid isn't seasonal, for example, it may call into question our current annual fall booster strategy, as I detail in a story out today. "Whether it will continue to have two peaks or just a single winter peak, we can't predict that," says John Vanchiere, a pediatrician and infectious disease specialist at Louisiana State University Health Shreveport. "We probably won't see a once-a-year peak until 5 to 10 years from now." —Cailley LaPara | 
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