Flu and COVID-19: How Do the Illnesses Compare?

The two share a lot of similarities, but there are key differences between them.

It's not uncommon for one or more symptoms to signal many different conditions or illnesses. When that happens, it may be hard to figure out what illness, disorder, syndrome, or infection a person is experiencing.

When COVID-19 was named a global pandemic in March 2020, it became important to figure out if someone had the flu or if they had COVID-19. Efforts to reduce the spread of COVID-19 resulted in "reduced transmission of some viral respiratory pathogens,"—including those that could cause the flu.

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.

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Getty Images / Jo Imperio

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 mainly infect a person's respiratory tract, said Nicolas Barros Baertl, MD, an infectious disease physician at Indiana University Health.

Past that, the two viruses also share some common symptoms, the ways that they spread, and how people can prevent getting both illnesses.

Symptoms

Because influenza and SARS-CoV-2 (the virus that causes COVID-19) are respiratory illnesses, they can both result in similar symptoms. A review noted that the most common symptoms seen in both illnesses are fever and cough.

However, the following symptoms may appear if you have the flu or COVID-19:

  • 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)

A person can also have either illness, showing a range of symptoms from no symptoms at all to several severe symptoms.

How the Viruses Spread

As with many respiratory illnesses, both COVID-19 and the flu spread mainly from person to person by infected respiratory particles, said Richard Zimmerman, MD, a professor of medicine and public health at the University of Pittsburgh and a family medicine physician.

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. 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.

And, although the potential for the illnesses to spread is more common when a person is already sick and showing symptoms, Dr. Zimmerman said 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
  • Spreading where people only show very mild symptoms

Prevention

Both COVID-19 and the flu have vaccines that prevent severe illness. No vaccine prevents illness 100%—but the approved vaccines and the ones authorized for emergency use greatly reduce hospitalization and death. Additionally, individuals aged six months and older are eligible to be vaccinated against the flu and COVID-19.

The flu has multiple US Food and Drug Administration (FDA)–licensed vaccines that are produced annually based on the four flu viruses experts believe will circulate. This is why flu vaccines are called quadrivalent influenza vaccines.

COVID-19 has four vaccines approved or authorized by the FDA, as of November 2022:

  • Pfizer-BioNtech
  • Moderna
  • Johnson & Johnson
  • Novavax

Other than 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," Dr. Zimmerman said. "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.

Influenza A viruses are further categorized into subtypes called 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.

Coronaviruses are a large family of viruses that typically cause mild to moderate respiratory tract illnesses, like the common cold. But COVID-19 is one of three coronaviruses—along with severe acute respiratory syndrome (SARS) and Middle East respiratory system (MERS)—that has caused severe illness on a global or very large scale.

Furthermore, flu symptoms might appear within one to four days after an individual has been infected. However, it may take up to 14 days for symptoms to appear if a person has been infected with COVID-19. Still, the differences between the two illnesses don't end there.

Complications

COVID-19 and the flu can cause many of the same symptoms. Additionally, both can lead to cases of:

  • Pneumonia
  • Respiratory failure
  • Acute respiratory distress syndrome (ARDS)

However, COVID-19 can affect the body's other organ systems more than the flu, said James H. Conway, MD, a pediatric infectious disease specialist and associate director for health sciences at the Global Health Institute of University of Wisconsin-Madison.

Although secondary bacterial infections, like sinus and ear infections, can occur in people with COVID-19 and the flu, it is more common to develop a bacterial infection from the flu.

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

Long COVID—sometimes known as 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 including:

  • Fatigue
  • Brain fog
  • Heart palpitations
  • Headache
  • Muscle aches

These symptoms can last weeks or months after being infected with the virus, even if you just had mild symptoms. Overall, "COVID-19 seems to do a lot more in the body to other organs than influenza," Dr. Conway said.

Infections and Deaths

There have been several hundred million confirmed cases and millions deaths due to COVID-19 globally, according to the World Health Organization's (WHO) COVID-19 dashboard.

In the US alone, COVID-19 has caused over 97 million confirmed illnesses and more than one million deaths. These are only confirmed cases—the actual number of COVID-19 cases may be much higher, since some people may not have seen a healthcare provider for potential symptoms.

In regard to the flu, 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.

As a 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%," Dr. Barros said.

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)
  • Xofluza (baloxavir marboxil)

These medications work best when they're started soon after flu symptoms begin—within approximately two days of symptom onset.

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 (e.g., sleep, fluids, and fever reducers). However, 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.

As of November 2022, there are two FDA-approved drugs to treat COVID-19: remdesivir (brand name Veklury) and baricitinib (brand name Olumiant).

According to the FDA, remdesivir is recommended for pediatric and adult patients who have tested positive for COVID-19 and are either hospitalized, or not hospitalized but have mild-to-moderate COVID-19 with high risk of progression to severe COVID-19.

Of note, pediatric patients must be at least 28 days old and weigh at least 3 kilograms (a little more than six and a half pounds) to be treated with remdesivir. Baricitinib is recommended for use in hospitalized patients who need supplemental oxygen.

The FDA has also granted Emergency Use Authorizations (EUAs) to a monoclonal antibody (mAB) product, or laboratory-produced antibody, called Bebtelovimab for mild and moderate cases of COVID-19. Other EUAs fall under the categories of antiviral drugs (like Paxlovid), immune modulators, sedatives, and renal replacement therapies for certain populations.

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," Dr. Zimmerman said.

A Quick Review

Overall, there are countless similarities and differences between COVID-19 and the seasonal flu. If you have any questions or concerns about either illness, including aspects such as symptoms or treatment options, you can talk with a healthcare provider for more information.

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