Because the Northern Hemisphere’s influenza season often mirrors that of the southern, American experts look south for clues as to what the coming flu season might look like.

And this summer, Australia’s flu season—which occurred during its winter—was shorter than normal. While hospitalizations and deaths were nothing unusual, cases of flu peaked earlier and higher. Oddly, children and teens, who usually fare well with the flu, bore the brunt. The majority of reported flu cases were among young people ages 0–14, according to an Oct. 9 report from the Australian government.

“We know the Australian flu season was pretty rough, especially on children,” Dr. Sarah Combs, an emergency medicine doctor at Children’s National Hospital in Washington, D.C., told Fortune. “We hope it won’t come to fruition here, but we expect it to and want to prepare for a rough flu season.”

Already Children’s National is seeing a steep increase in flu cases, with positive tests doubling every week since early October. While it’s only seen 80 flu patients since July, that’s 10 times more patients than the hospital saw during the same period last year, she said—”and we’re not even into winter.” Multiple children’s hospitals in Maryland, Virginia, and Washington, D.C., including Children’s National, were operating at or near capacity this week, according to D.C.’s NPR-affiliated station.

The number of positive flu cases reported to the U.S. Centers for Disease Control and Prevention has been rising steadily since September. Usually, cases don’t begin picking up until October.

The CDC has genetically sequenced nearly 400 flu virus samples collected since May. More than half occurred in those ages 0–24, according to the agency’s data. Only around 7% of cases were among people 65 and older, in contrast with many other years, in which the elderly were among the hardest hit.

Why children? And why this year?

Why did Australian children have a rough go of it this flu season? And why is the U.S. seemingly following the same pattern?

Some argue that children have been exposed to fewer viruses during the past couple of pandemic years, rendering their immune systems weaker than they would have been otherwise. Others argue that record-low levels of COVID precautions that would otherwise help keep the flu at bay will contribute to this winter’s surge among all populations, and especially among school-age children.

Still others speculate that COVID weakens the future immune response of at least some people it infects, leaving them less able to fight off all viruses. And yet others surmise that something about this year’s predominant strain of flu—Type A—may cause it to hit children unusually hard.

“All of the above might be at play,” Combs said. “COVID-19 is still new to us and we’re still learning, but we think maybe there’s an immune system effect, maybe there’s a long COVID effect. We don’t quite know.”

An additional concern of public health officials is that COVID vaccine hesitancy has spilled over into other areas.

“There’s a bit of vaccine phobia from COVID,” Dr. Georges Benjamin, executive director of the American Public Health Association, told Fortune. “There’s a concern that people may not get annual flu shots as well.”

And if an early start to the flu season results in an early peak, moving it up from January/February to November/December, spread could be all the more rampant due to holiday gatherings, Benjamin added.

“Somewhere between ‘normal bad’ and ‘H1N1 bad’”?

The issue of age aside, flu season is likely to be severe this year, experts say. But severe in what way? A mountain of cases that are uncomfortable but manageable at home? A rise in hospitalizations and death? Or both?

Benjamin says he expects a “robust” flu season—but that doesn’t necessarily equate to increased hospitalizations and deaths, just as it didn’t this year in Australia.

At the very least, Combs anticipates a flu season worse than last year—10 times the number of flu cases as last year, in fact. In 2021 and 2020, the U.S. and other countries saw record low levels of flu due to COVID precautions.

And at the very worst, she anticipates a situation similar to the 2009 H1N1 flu pandemic—the first flu pandemic of the 21st century. It infected millions, if not billions, worldwide, and killed more than 18,000. Children and young adults also bore the brunt of flu cases that year.

“We did have a couple of rough flu seasons leading up to the pandemic,” she said. “Maybe we will settle out around these, somewhere between ‘normal bad’ and ‘H1N1 bad.’”

The COVID complication

Then there’s the matter of COVID, which will inevitably complicate this flu season, and flu seasons to come. The fact that flu and COVID will likely peak at the same time is a “central challenge we have as a nation,” Benjamin said. Hospital capacity could end up being strained by the potential “twindemic.”

“The signs and symptoms for influenza are the same as COVID, with the exception of losing taste and smell” with COVID, he said. “If someone has malaise, fever, chills, muscle aches, headache, maybe people tend to say, ‘I have the flu’ when they have COVID, or ‘I have COVID’ when they have the flu.”

The best advice in both cases is to “stay home if you’re sick,” Benjamin added. “We have diagnostic tests for both. Vaccines for both. And antivirals for both. We have the tools to minimize this substantially if we use them.” But confusion could cause some to downplay the risk of both.

To add insult to injury, Children’s National is currently seeing a “crazy high” number of influenza-like illnesses such as rhinovirus and RSV, and patients are sicker than usual from such viruses, according to Combs. Her observations this fall mimic those of ER and critical care physicians throughout the country, she says.

Whatever the nature of coming waves of flu and COVID, one thing is certain, biologist Dr. Ryan Gregory, a professor of evolutionary biology at the University of Guelph in Ontario, Canada, told Fortune: The added challenge of flu this year, on top of COVID, “is certainly not going to make things better.”

“The simplest version is that even a moderately bad flu season and an equivalently bad COVID wave is still a bad winter.”