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December 2, 2020 12:43 PM EST

Every winter is a bit of a roulette wheel when it comes to influenza. Flu vaccines work, but aren’t 100% effective in preventing disease, so it’s always a challenge convincing people to get their flu shots. And while the symptoms are generally bearable, infections can become more severe and even deadly among people who are older or who have underlying health conditions. Last flu season, even though experts considered it a relatively mild year, about 400,000 people in the U.S. were hospitalized and 22,000 people died from the flu.

This winter, the influenza virus has a rival—the coronavirus fueling the COVID-19 pandemic—and health officials are anticipating a showdown that could have dire consequences for the health of millions. Both diseases are caused by viruses that spread with abandon from person to person through sneezes, coughs, and respiratory droplets during close contact. But while researchers know quite a bit about the influenza virus, the coronavirus, SARS-CoV-2, is very much a black box, and they can only guess what will happen when the two pathogens collide throughout the world.

Here is a rundown of what to expect and how best to protect yourself this winter.

Why is it dangerous to have flu and COVID-19 viruses around at the same time?

A double whammy of flu and SARS-CoV-2 infections this winter could be devastating for public health, as the colder weather brings people into closer contact with each other in confined spaces indoors. “What I worry about is that both viruses can cause serious respiratory illness,” says Dr. David Chokshi, health commissioner for New York City. “In the same way that COVID-19 often gets to the point where someone has to be hospitalized to support their breathing, or needs to be in the ICU with a breathing tube or ventilator, unfortunately we see that in the most severe cases of influenza as well. What we worry about is that happening at the same time and really stressing the capacity of our hospitals.”

Public health experts are concerned that too many people who are seriously ill with influenza or COVID-19 could flood hospitals and stretch already worn out health care workers, and health care systems, to their limits.

What can be done to prevent a dual-virus disaster?

One of the most important things a person can do to ensure the flu season doesn’t turn into a public health catastrophe is to get a flu shot. And even though it’s still early in the flu season, health experts are already urging people to do just that. While there are only a few therapies for treating COVID-19, there are drugs and a vaccine that can protect people against influenza infection, even if they’re not perfect. The flu vaccine is typically up to 50% effective, but that 50% is better than no protection, which is essentially where most of the world’s population stands now with respect to COVID-19. “For influenza, we have a vaccine. It’s one extra layer of protection for the public that could help reduce cases,” Dr. Andrew Pekosz, co-director of the Johns Hopkins Center of Excellence for Influenza Research and Surveillance, said during a recent briefing.

Public health officials are putting extra effort into flu shot campaigns this year, but those programs have been hampered by the fact that many of their traditional partners in providing flu shots, such as employers, schools and faith-based organizations, are shut down or only reaching a limited number of people due to social distancing measures.

The good news, however, is that so far, some of the signs health officials watch for predicting the severity of a flu season are suggesting a lower number of cases this year. Because the U.S.’s winter follows that of the southern hemisphere, infectious disease experts look to countries like Australia, South Africa and Chile, for hints about the upcoming flu season. This year, possibly because COVID-19 has caused lockdowns and reduced social contact, and led to more people wearing masks in public, cases of influenza there have been lower than in previous years. “They experienced hardly any influenza virus activity,” says Dr. Tim Uyeki, chief medical officer at the Centers for Disease Control’s influenza division.

Right now, there is low flu activity in the U.S., and while the trends in the other parts of the world bode well, there are also some worrying signs. In parts of southeast Asia and south Asia, some countries are seeing an uptick in influenza cases. Specifically, they’re seeing more of an influenza A virus known as H3N2, which tends to cause more severe illness than other flu strains. “What’s unknown is whether the increase in influenza activity in southeast Asia will carry over to other countries, and what the implications are for the U.S.,” says Uyeki.

Can I get flu and COVID-19 at the same time?

Yes, says Uyeki. “There is no question that co-infection can occur; we’ve seen case reports and case series from a number of countries.”

What’s less clear is whether being infected with both influenza and SARS-CoV-2 means people will get less, or more sick than those battling just one viral infection. And because COVID-19 is such a new disease, it’s also too early to tell whether being infected with one virus, recovering, and then getting infected with the other can affect the course of disease. “We don’t know what is going to happen to co-infections here in the U.S.,” Pekosz said. “But there is some data that we are concerned about that one virus may be causing an environment in the lungs that makes it easier for the other virus to infect. There is much speculation about that right now, and it’s serious enough for us to pay attention to co-infections and monitor them carefully as we enter the winter season.”

That’s another reason why doctors are especially adamant about asking people to get their flu shots, since it’s possible the viruses may be interacting in ways that could compromise people’s health but aren’t widely known yet.

How will I know if I have the flu or COVID-19?

You likely won’t, and neither will your doctor, unless you get tested. The symptoms for both infections are similar enough—fever, cough, muscle aches—that only a test will distinguish between the two viruses. During most flu seasons, most doctors generally don’t test their patients for influenza since it’s almost always the most common respiratory virus making the rounds. But this season is different, and more doctors may be ordering flu, COVID-19, or combination tests to make an accurate diagnosis, since isolation and quarantine are particularly important for people infected with COVID-19 to prevent the spread of that disease.

“The fact that the two viruses exist together [this season] means there are much higher stakes for actually making a diagnosis, and knowing whether it’s influenza or SARS-CoV-2,” says Dr. Jay Wohlgemuth, chief medical officer at Quest Diagnostics, one of the large commercial testing labs in the country.

Indeed, many flu test makers are offering a combination influenza and SARS-CoV-2 test that will help doctors to diagnose both viral illnesses with just one swab from a patient’s nose or throat—and demand for the combo tests is already increasing, says Dr. David Persing, chief medical and technology officer at Cepheid, another testing lab. “Demand has increased several fold from our requests for influenza tests,” he says. So much so that the company is focusing more resources on making the dual tests than on producing COVID-19 tests. “We can’t afford to not be ready for this.”

Will a flu vaccine protect against COVID-19?

No, flu vaccines and COVID-19 vaccines target two different viruses—influenza and SARS-CoV-2, respectively. Each is designed to specifically train the immune system to recognize and launch attacks on its particular virus.

Is it dangerous to get both the flu vaccine and the COVID-19 vaccine?

There’s no reason to believe that the two vaccines will interact with each other and cause any adverse events. But no COVID-19 vaccines have been authorized by the U.S. Food and Drug Administration yet, although the agency is now reviewing two potential vaccines and will likely make a decision about authorizing them by the end of December. Authorization, however, doesn’t clear up the remaining unknowns about how exactly the vaccines trigger the immune system. Researchers will be following people who participated in the trials of the experimental COVID-19 shots for two years to see, among other things, how they react to the flu shot.

Until this year’s flu season is behind us—which won’t be until next March or April—doctors won’t know how well their efforts to urge people to get their flu shots were in controlling not just influenza but COVID-19 as well. Until then, they remind everyone that the same practices can reduce risk of getting infected with both viruses: washing your hands, wearing a mask, staying at least six feet away from others not in your household, and avoiding public gatherings.

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