What happens if you catch flu and COVID-19 at the same time?

It’s a familiar rite of autumn: Neighborhood pharmacies hang their “Flu Shot Available Here” signs. Primary care providers fill our email inboxes with reminders to get our annual vaccinations.

This year is different. We’re facing a “twindemic” of both the seasonal flu and COVID-19, and research shows it’s possible to get sick with both respiratory illnesses at the same time. Unfortunately, history proves people haven’t done a good job of taking the flu seriously enough. Just 45 percent of adults in the United States got the flu shot last year, according to the Centers for Disease Control and Prevention, even though the seasonal version of the disease is deadly, killing an average of 37,000 people every year over the past decade. Having both diseases in play simultaneously threatens to tax our health-care systems and puts lives at risk like never before. That’s not to mention the real possibility that a strain of bird flu lurking at poultry markets could jump to humans and cause an overlapping pandemic.

National Geographic asked two infectious disease experts to weigh in on what’s at stake with this year’s dual threat, and why we all should get vaccinated against the flu as early as possible. The following interviews have been edited for length and clarity.

Getting COVID-19 is significantly worse than the flu. So why are public health officials suddenly worried about flu shots?

Lisa Maragakis, senior director of infection prevention at Johns Hopkins University School of Medicine in Baltimore, Maryland:

The symptoms for flu and COVID-19 are so similar that one of the challenges we’re dealing with this year is diagnosing people correctly and quickly. Even if you have mild symptoms, don’t attempt to ride out a virus on your own, and don't assume that coughing is the only clue you've got COVID-19. You should contact your doctor if you have body aches, fever, a sore throat, or respiratory symptoms so you can be tested for COVID-19. The list of warning signs for the coronavirus is continually expanding and now includes loss of taste or smell, nausea, diarrhea, or even swollen red toes.

It’s important to know which infection you have. With the flu, your doctor can prescribe an antiviral medication. But if you have COVID-19, your doctor will help you decide if you need to go to the hospital for severe symptoms where you might be prescribed steroids or other experimental medications. Plus, you’ll have to be quarantined to avoid transmitting it to others.

You might not think the flu is a big deal if you get a mild case, where you feel under the weather for a few days and your symptoms go away on their own. But just ask someone who’s recovered from viral pneumonia caused by the flu to understand how miserable you can be. It can also land you in the hospital. I don’t think people appreciate how severe it can be.

What should we expect from the northern flu season, given what happened in the Southern Hemisphere this year?

Robert Webster, infectious disease specialist at St. Jude Children’s Research Hospital in Memphis, Tennessee:

No one can ever predict how severe a flu season will be. But there has been some good news from southern countries, such as Australia, New Zealand, and Chile, where the flu season is just ending. The rates of flu there were surprisingly low. The reason is that these governments implemented such effective strategies to control the coronavirus—social distancing, hand washing, and mask-wearing—that the flu didn’t get the chance to take hold. Melbourne, Australia, for example, is again under lockdown.

Since children are typically the biggest spreaders of flu, this means school closures also will help stop transmission. Don’t get a false sense of confidence—even if you’re being careful and following your local health guidance. If you’re a responsible human being, you still need to get a flu shot, because you could pass the disease on to a more vulnerable person who could die from it.

Some people think the flu vaccine doesn’t work very well and isn’t worth the trouble. What would you tell them?

Maragakis: The flu vaccine is never 100 percent effective, and the amount of protection it offers varies from year to year. It’s made many months in advance and is composed of four different strains that are predicted to circulate during flu season.

The flu vaccine is not perfect, but it’s the best prevention that we have, and we should take advantage of it. Even if you get the flu after being vaccinated, you’re more likely to have a less severe case and recover quicker.

Where can people go to safely get the flu vaccine? Is it worth taking the risk if you're at high risk for COVID-19?

Maragakis: The flu vaccine is widely available at clinics, pharmacies, and supermarkets, so people have several choices as to where to get it. If you’re worried about standing in line with a crowd because you don't want to be exposed to COVID-19, it’s worth calling your local pharmacy or health-care provider to ask about setting up an appointment. Many medical centers have been taking precautions to keep people safe, such as requiring masks and physical distancing or doing extra cleaning. Despite the myth that vaccines can make you sick, a flu vaccine won’t give you the flu, even if you’re immuno-compromised.

What happens if you catch the flu and COVID-19 at the same time? Is it worse to contract one before the other?

Maragakis: We know you can get two viral illnesses at once, and that it’s harder for your immune system to fend them off at the same time. You’re at risk for severe lung damage, a longer illness, worse complications, and even death.

We don’t know yet if having COVID-19 or flu increases your chances of getting the other. But you’re definitely more vulnerable, because viral infections damage tissue in your respiratory tract, so you’ll have a harder time fending off subsequent ones. Having inflamed lungs [which either disease can cause] also offers the opportunity for bacteria to make its way in there.

Why should people get their flu shot early?

Maragakis: I encourage people to get their flu vaccine by the end of October. It takes 10 to 14 days from the time you get the vaccine to develop immunity, so getting it early gives you the best protection before the flu season gets in full swing. The longer you wait, the more you increase your chances of catching the flu.

Do you think a universal flu vaccine will ever be a reality?

Webster: That’s my dream. The advantage of a universal vaccine is that it would work against multiple strains of flu, so patients wouldn’t have to get a new shot every year. It would also eliminate the need for researchers to try to guess which strain will circulate every year and offer more thorough protection, especially from newly emerging threats.

The science is promising, and I believe we’re only five years away from it being a reality. In fact, there are several universal vaccine candidates in phase three trials right now. They’re producing effective antibodies that target the stalk of the flu virus, which is the same across 18 flu subtypes.

Are we better prepared this year for a new pandemic flu strain?

Webster: Pandemics can’t be predicted, and we have no way of knowing how a bird or swine flu can develop the ability to be transmittable to humans. It’s a combination of random genetic mutations and how the virus interacts with the human immune system. There are two kinds of viruses in poultry markets in Asia right now that worry me for their potential to jump to humans.

But COVID-19 forced us to ramp up our vaccine manufacturing capacity and explore novel ways of making vaccines. There are more than 150 companies working on a COVID-19 vaccine. Having this infrastructure and a culture of creative thinking in place will benefit us greatly, because we’ll be able to apply the lessons of this pandemic to the next one.

It’s inevitable there will be another outbreak in our future. But we’ll be better prepared. That fact gives me hope.