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 indicate the possibility of multiple conditions or illnesses. When symptoms mark multiple conditions, it may be hard to determine an illness, a disorder, a syndrome, or an infection at face value.

For example, if a person has a headache on its own, it could signify things such as being hungry or stressed. However, if they have confusion along with the headache, it's possible that they could be experiencing dehydration, a stroke, a brain infection, or meningitis—calling for a more thorough evaluation for diagnosis and treatment.

When COVID-19 was deemed a global pandemic in March 2020, it later became important to distinguish between someone having the flu and COVID-19. Efforts to reduce the transmission of COVID-19 resulted in "reduced transmission of some viral respiratory pathogens," per a 2021 Morbidity and Mortality Weekly Report—including those that could cause the flu. However, the two illnesses could present the same way initially due to having overlapping symptoms.

Finding out which of the illnesses you might have or if you have a combination of both requires detailed knowledge of the flu and COVID-19, including being aware of the similarities and differences regarding symptoms, causes, treatment options, and how they spread.

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Getty Images / Jo Imperio

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 mainly infect a person's respiratory tract, Nicolas Barros Baertl, MD, an infectious disease physician at Indiana University Health, told Health. Past that, the two viruses also share some common symptoms, modes of transmission, and prevention techniques.

Symptoms

Because influenza and SARS-CoV-2 (the virus that causes COVID-19) are respiratory illnesses, they can both result in similar symptoms, per the Centers for Disease Control and Prevention (CDC). A 2020 review published in Reviews in Medical Virology noted that the most common symptoms seen in both illnesses are fever and cough. However, the CDC says 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 according to the CDC.

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, a professor of medicine and public health at the University of Pittsburgh and a family medicine physician, told 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 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 in people only showing 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 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, according to the CDC.

Similarly, COVID-19 has four FDA-approved vaccines available for use, as of August 2022:

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

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," 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, per the CDC. 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—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 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 longer for symptoms to appear if a person has been infected with COVID-19. Still, 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, a pediatric infectious disease specialist and associate director for health sciences at the Global Health Institute of UW-Madison, told Health.

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

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 in flu cases. The illness can also lead to multisystem inflammatory syndrome in children (MIS-C), 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—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 said.

Infections and Deaths

As of August 2022, there have been more than 596 million confirmed cases and nearly over six million 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 nearly 93 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, 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.

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), 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 (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, the CDC says.

As of August 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 weight at least 3 kilograms (a little more than six and a half pounds), per the FDA, 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 two monoclonal antibody (mAB) products—one combination mAbs (Evusheld, which is tixagevimab co-packaged with cilgavimab) and a single mAb (Bebtelovimab)—for those with mild to moderate COVID-19 symptoms and risk factors for the progression of the disease. Other EUAs follow 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.

Overall, if you have any questions or concerns about either illness, including about aspects such as symptoms or treatment options, you can tlalk with your healthcare provider for more information.

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