Now is about the time that the annual flu shot reminder articles start popping up. It’s rarely on people’s minds in late August or even in September, but that’s actually the perfect time to get vaccinated. It takes a few weeks for immunity to build, so by the time the virus really starts circulating you’re already protected.

This year, though, we all have another disease on our minds, and the growing awareness of two overlapping, widespread viruses circulating at the same time has epidemiologists and other public health experts concerned.

“I’m very worried about this,” says Eleanor Murray, an epidemiologist at Boston University School of Public Health. “I would feel more secure if we were taking strong COVID-19 precautions, but with schools and universities and many workplaces opening back up, I’m not at all optimistic about the fall and winter.” Here are the reasons Murray and others are so concerned.

COVID symptoms might be mistaken for the flu

There’s a lot of overlap between the symptoms that each virus causes. Both can give you a fever, cough, fatigue, sore throat, stuffy nose, muscle pains, shortness of breath, and headaches. But the outcomes of each disease are very different—COVID-19 is much deadlier than the flu (some evidence suggests as much as 50 to 100 times deadlier). One reason for that is that very few people have any kind of immunity to it.

The current advice for people who think they might have COVID-19 is to self-isolate, then go to a hospital if they start exhibiting serious symptoms like shortness of breath which could be life threatening. . But as flu season ramps up, folks who come down with COVID-19 symptoms might brush it off as “just the flu.” They might not take proper precautions and then spread the coronavirus to others. They also might be less willing to see a doctor for the flu, which could put them at a higher risk of experiencing a dangerous case of COVID-19.

Already overloaded testing systems are going to get even farther behind

The logical way to differentiate influenza from COVID-19 would be to get tested. Self-isolation in combination with easily available testing could be an excellent way to tamp down both viruses—according to epidemiologists, proper testing is a big factor in helping us return to relative normality.

But the United States doesn’t currently have that capability. We were already behind on COVID-19 testing in February, and in the first two weeks of August we ran fewer tests than the week before. Nonprofits, philanthropists, and healthcare professionals have tried filling in the gaps where the government has largely failed, but it will take time for companies to get systems in place to help everyone who needs it. And meanwhile, the current testing system is falling farther behind. Often, results take days or even weeks to get back, which isn’t a useful timeframe when people are already infectious before they’re symptomatic.

So when a flood of people start getting COVID and flu-like symptoms and want to get tested to see if they’ve been infected with the novel virus, the system isn’t going to be able to help them. “If people are going to get tested or self-isolating for any fever then the testing systems are going to become overloaded,” Murray says. Testing is the most basic form of pandemic control—it enables us to isolate the sick from the well—and it’s not a good sign that we’re already behind.

Healthcare systems could get overloaded, too

On top of a breakdown of the testing systems, there’s a serious concern that an influx of people with severe influenza could push healthcare systems over the edge too. Despite much drum-beating on the topic, much of the public is still unaware that seasonal flu is a serious affair.

“One of the common refrains we’ve heard from people is ‘isn’t COVID-19 just like the flu, and therefore no big deal?’ but the flu is actually a pretty big deal,” Murray says. “A lot of people get sick and die from it every year and this year is likely to be no different. Add that on top of COVID-19 and our health care and public health systems which are already overtaxed are likely to completely break down.”

Co-infections might be an issue

Diseases usually strike one at a time, but it is possible to get infected with two viruses at once. Having influenza and COVID-19 at once—both respiratory viruses—would be nightmarish. The real question is how likely that is to actually occur.

Co-infections are pretty rare in general, and in some cases being infected with one virus can have a protective effect against others (for instance, having influenza A seems to reduce the chances of having a rhinovirus), though virologists aren’t exactly sure why that is.

“I do think it’s reasonable to be concerned that co-infections could be more deadly, although we have no evidence either way as far as I’m aware,” Murray says. Unfortunately, says Murray, this is something we’ll have to wait and see on.

The good news: COVID precautions are also influenza precautions

One small glimmer of hope comes from the southern hemisphere: Flu cases were at historic lows this year. Australia, which has already been through the worst of this year’s winter season, had 925 cases in 2018, then 9933 last year, but in 2020 only saw 33. A similar pattern has occurred in Argentina, Chile, and South Africa. All of those countries had over a thousand cases in 2019 and at least 700 the year before, but have had less than 55 cases this season.

Though it might be that people are simply not showing up to hospitals and therefore not being counted, it’s more likely that those countries ended up with low flu counts due to the social distancing measures they put in place for COVID-19. “In general, we’re very good at doing flu surveillance so I believe that the reduction in flu is real,” says Murray. “It seems reasonable to attribute that to the precautions we are taking against COVID-19 also being effective for the flu.” The US was at the tail end of what should have been a severe flu season in 2020, but when the pandemic was declared there was a precipitous drop in cases.

But that doesn’t mean this winter is going to be an easy one.

“It’s important to note that the precautions that many Southern hemisphere countries have been taking against COVID are stricter than the precautions we are taking in the US,” Murray notes. “We can’t necessarily rely on seeing a similar decrease in flu here."

Written by Sara Chodosh for Popular Science and legally licensed through the Matcha publisher network. Please direct all licensing questions to legal@getmatcha.com.