Last year, during the 2020–2021 flu season, the US saw record low numbers of influenza illnesses-between October 3, 2020 and July 24, 2021, the CDC saw only 2,136 positive flu tests out of 1.3 million specimens tested by clinical laboratories, according to official data provided to JAMA. Meanwhile, COVID-19 continued to tear its way through the US-during the same time frame, there were 27.1 million confirmed cases of COVID-19 in the US, and more than 401,000 deaths, according to the Johns Hopkins Coronavirus Resource Center.

But the outcome may be a little different this year-and that worries experts. Earlier in October, Rochelle Walensky, MD, MPH, director of the Centers of Disease Control and Prevention, warned of a "potentially severe flu season this year," during a White House press briefing. Dr. Walensky added that the decreased masking and physical distancing guidance, paired with lower "population-level immunity" to the flu, is what might make this season more severe.

While the best way to protect yourself against both illnesses is through vaccination and other prevention techniques like wearing a mask in highly-crowded areas, it's also wise to arm yourself with knowledge on the flu and COVID-19. Knowing the similarities and differences between the viruses-including symptoms, causes, treatment options, and how they spread—is a good place to start. Here, infectious disease experts weigh in on how the flu and COVID-19 compare, to help you take care of your health and protect others in your community.

What are the similarities between COVID-19 and the flu?

COVID-19 and the flu share many overlapping characteristics. First and foremost, they're both primarily respiratory illnesses, meaning they mostly infect a person's respiratory tract, Nicolas Barros Baertl, MD, an infectious disease physician at Indiana University Health, tells Health. Past that, the two viruses-influenza and SARS-CoV-2-also share some common symptoms, modes of transmission, and prevention techniques.

Symptoms

Because both influenza and SARS-CoV-2 (the virus that causes COVID-19) are respiratory illnesses, they share many symptoms. Both illnesses can result in flu-like symptoms, per the Centers for Disease Control and Prevention (CDC), which include:

Fever and chills
Cough
Shortness of breath or difficulty breathing
Fatigue
Sore throat
Runny or stuffy nose
Muscle pain or body aches
Headache
Vomiting and diarrhea (more common in children with the flu or COVID-19)
Change in or loss of taste and smell (more common with COVID-19)

How they spread

As with many respiratory illnesses, both COVID-19 and the flu spread mainly from person-to-person by infected respiratory particles, Richard Zimmerman, MD, MPH, a professor of medicine and public health at the University of Pittsburgh and a family medicine physician, tells Health. Transmission happens most commonly with close contact with infected people when a sick person coughs or sneezes out infected droplets that can then be inhaled by another nearby healthy person, the CDC says.

While spread largely happens between direct contact with people, both viruses also have the potential to spread by touching another person or object that has the virus on it, per the CDC.

And, although spread is more common when a person is already sick and showing symptoms, Dr. Zimmerman says both viruses are also capable of asymptomatic spread (when someone is infected but doesn't show symptoms), pre-symptomatic spread (before a sick person begins to show symptoms), and in people only showing very mild symptoms.

Prevention

Both COVID-19 and the flu have vaccine that prevent severe illness. Keep in mind: No vaccine prevents illness 100%—but the approved vaccines and the ones authorized for emergency use greatly reduce hospitalization and death.

Per the CDC, the flu has multiple Food and Drug Administration (FDA)–licensed vaccines that are produced annually based on the four flu viruses experts believe with circulate (that's why flu vaccines are called quadrivalent influenza vaccines.

Similarly, COVID-19 has three vaccines available for use—the FDA-approved Pfizer-BioNtech COVID-19 vaccine, which consists of two shots spaced three weeks apart; the Moderna vaccine, which is also two shots spaced apart; and the Johnson & Johnson/Janssen single-dose vaccine. The FDA and CDC also recently approved booster doses for certain populations for all three vaccines.

Past vaccines, you can also help prevent both illnesses with proper prevention techniques like wearing masks, washing your hands frequently, and physically distancing from large groups of people. "Masking can make a real difference for both COVID-19 and flu," says Dr. Zimmerman. "That's part of why influenza basically disappeared in 2020."

What are the differences between COVID-19 and the flu?

Though both viruses primarily cause respiratory illnesses, the viruses are not the same. The flu is caused by influenza viruses, of which there are two main types that affect humans: influenza A viruses and influenza B viruses, per the CDC. Influenza A viruses are further categorized into subtypes (hemagglutinin (H) and neuraminidase (N), of which there are numerous combinations), and influenza B viruses are broken down into lineages (B/Yamagata and B/Victoria).

COVID-19, on the other hand, is caused by the SARS-CoV-2 virus. The virus is technically called a coronavirus-and coronaviruses are a large family of viruses that typically cause mild to moderate respiratory tract illnesses, like the common cold, according to the National Institute of Allergy and Infectious Diseases (NIAID). But COVID-19 is one of three recent coronaviruses-along with severe acute respiratory syndrome (SARS) and Middle East respiratory system (MERS-that can cause severe illness on a global or very large scale.

The differences between the two illnesses don't end there-here are a few more ways COVID-19 differs from the flu.

Complications

Though COVID-19 and the flu can cause many of the same symptoms, and both can lead to cases of pneumonia, respiratory failure, and even acute respiratory distress syndrome (ARDS), COVID-19 can affect the body's other organ systems more than the flu, James H. Conway, MD, FAAP, a pediatric infectious disease specialist and associate director for health sciences at the Global Health Institute of UW-Madison, tells Health.

The CDC notes that secondary bacterial infections, like sinus and ear infections, from the flu are more common.

In COVID-19 infections, the CDC says blood clots in the veins or arteries in the lungs, heart, legs, or brain are more common than with flu cases. The illness can also lead to something called multisystem inflammatory syndrome in children (MIS-C), in which various parts of the body become inflamed (heart, lungs, kidneys, brain, skin, eyes, gastrointestinal organs).

Long COVID-sometimes known a long-haul syndrome or post-COVID-is also a complication of COVID-19 not seen in flu cases. This syndrome includes a range of symptoms-fatigue, brain fog, heart palpitations, headache and muscle aches, among others-that can last weeks or months after being infected with the virus, even if you just had mild symptoms, the CDC says. Overall, "COVID-19 seems to do a lot more in the body to other organs than influenza," Dr. Conway says.

Infections and deaths

Since its emergence in Wuhan, China in December 2019, there have been more than 240 million confirmed cases and nearly five million deaths, globally, due to COVID-19, according to the World Health Organization's (WHO) coronavirus dashboard. In the US alone, COVID-19 has caused nearly 45 million confirmed illnesses and more than 726,000 deaths. Keep in mind, these are only confirmed cases-the actual number of COVID-19 cases may be much higher, since some people may not see a doctor for potential symptoms.

In regards to the flu, the WHO says that, globally, there are an estimated one billion cases each year, and up to 650,000 influenza-related deaths each year. The CDC says the estimated annual burden of the flu in the US specifically is between nine million and 41 million illnesses, and 12,000–52,000 deaths each year

On the whole, COVID-19 is much more deadly than the flu, and more likely to result in severe illness and hospitalization. "Compared to the 0.01% case fatality rate for influenza, the case fatality rate for COVID-19 is 0.2%," says Dr. Barros.

Treatments

The flu can be treated with prescription medications known as antiviral drugs—the FDA has approved four of them: Tamiflu (oseltamivir phosphate), Relenza (zanamivir), Rapivab (peramivir), and Xofluza (baloxavir marboxil), the CDC says. These medications work best when they're started soon after flu symptoms begin—within approximately two days of symptom onset, per the CDC. Not everyone needs antiviral drugs for the flu: Most people who are healthy and not at risk for complications can likely treat flu with the typical home remedies (sleep, fluids, fever reducers); but those who have severe illness but don't need hospitalization, or people with mild illness who are at high risk for complications may benefit from antivirals, the CDC says.

There is only one FDA-approved drug to treat COVID-19 as of right now: remdesivir. According to the National Institute of Health's (NIH) COVID-19 Treatment Guidelines, it's recommended for use in hospitalized patients who need supplemental oxygen. The FDA has also granted Emergency Use Authorizations (EUAs) to three monoclonal antibody (mAB) products—two combination mABs (bamlanivimab plus estesevimab and casirivimab plus imdevimab) and a single mAB (sotrovimab)—for those with mild to moderate COVID-19 symptoms and risk factors for the progression of the disease.

Something to consider: While the antiviral medications for the flu are relatively easy to get, treatment options for COVID-19 are much more complicated. "It's not quite the same as dropping by your doctor's office to get a prescription," says Dr. Zimmerman.