As the nation prepares to enter another uncertain peak season of COVID-19, health experts have begun raising the alarm on what they’re calling a possible “twindemic” – the chance of a particularly severe flu season overlapping with rising COVID-19 cases.

It’s not the first time they’ve sounded that particular alarm, of course. But last year, following months of dire warnings, the prevalence of the flu in the U.S. and around the world was abnormally low, according to data from the Centers for Disease Control and Prevention. Public health officials attributed the low case numbers to the social distancing, masking and lockdown precautions already in place due to COVID-19.

But this year could prove different. With many people in the U.S. fully vaccinated against COVID-19, pandemic-fueled restrictions are generally looser than last year. That means the flu season will likely be unpredictable.

“It’s complicated,” said Dr. Anna Bershteyn, assistant professor of population health at NYU Grossman School of Medicine. “It’s possible things could get much worse than in the past, or we could have another year that’s much better than in the past. It really depends on our choices and our behavior, but even then there’s a lot of uncertainty.”

Bershteyn noted there are several factors working in favor of a mild season. In the pandemic’s wake, flu-minimizing practices such as hand-washing, mask-wearing and social distancing have become ingrained.

“If you had asked people before COVID-19 if we could make the flu season go away by just changing our behavior, I don’t think most people would have thought we could,” Bershteyn explained. “It completely stopped flu in its tracks, in both strict and relaxed lockdowns. That tells us flu is even more fragile than COVID-19 – that we can disrupt it.”

The main problem, however, is that with social isolation comes weakened flu immunity. Experts warn that nearly 20 months of preventative measures has created a greater risk for people to fall ill if they do actually encounter the flu.

“This is an experiment no one has ever done before,” Bershteyn said. “What happens when you take an entire population and you don’t give them any natural immunity to flu? Certainly that can make the next flu season a lot worse.”

It’s worth remembering that flu season sucks up hospital and healthcare resources, just as COVID-19 does. Typically, between 9 and 45 million people in the U.S. become infected with the flu annually, the CDC reports. That includes up to 810,000 hospitalizations and 61,000 deaths.

Given the burden of the Delta variant, which has led hospitals to postpone non-COVID-related procedures, as well as potential workplace shortages with more hospital vaccine mandates, a “twindemic” would have a snowball-like effect.

Still, Bershteyn is hopeful that maintaining the behaviors we’ve adapted during the pandemic, accompanied by an increased push for flu vaccination – will help stave off the worst-case scenario, at least this year.

“If I had to guess how things would play out in that balance of pluses and minuses, I would say as long as we continue to mask and have precautions, we may be able to prevent a severe flu season,” she said. “When we do go back to those normal behaviors in full, that’s the point when flu is going to hit us – and it’s going to hit us harder than normal. If we choose to not go back to those behaviors in full anytime soon, we can prevent that really bad wave from hitting us.”

Despite confusion over COVID-19 booster shots, Bershteyn stressed that healthcare providers can provide flu and booster shots in a single visit.

That, of course, still leaves the world with a long-term problem: With exposure to the flu remaining low, immunity would still wane. The only way out of that problem is through vaccination, Bershteyn noted. That’s why researching and developing a pan-flu or universal flu vaccine will be crucial in the coming years.

“I hope for the best, but I also hope that even if we get the best outcome, that people won’t stop listening to warnings and that we don’t lose faith in scientists,” Bershteyn said.