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Flu or COVID-19?

Aug 12, 2021 By William Kimbrough

Updated August 9, 2021.

As fall approaches, so too does the start of the flu season. But unlike other years, this season comes as the U.S. continues to fight the spread of another contagious respiratory illness: COVID-19. Complicating matters is the fact that both illnesses share similar symptoms, making it difficult to distinguish one from the other. Understanding the similarities and differences between COVID-19 and the flu can help you better prepare and protect yourself this fall. Here’s what you need to know to stay healthy:


Both COVID-19 and the flu can vary in terms of degree and severity of symptoms from person to person. Some may have more severe cases of these illnesses, while others may be completely asymptomatic. Shared symptoms of both the flu and COVID-19 include the following:

  • Fever or feeling feverish/chills
  • Cough
  • Shortness of breath or difficulty breathing
  • Fatigue
  • Sore throat
  • Runny or stuffy nose
  • Muscle pain or body aches
  • Headache
  • Vomiting or diarrhea

Unlike the flu, however, some people with COVID-19 may experience new loss or taste or smell. Additionally, while flu is associated with a more rapid onset of symptoms, one to four days after infection, symptoms of COVID-19 may take between 2 to 14 days to develop. Typically, a person starts showing symptoms 5 days after being infected.


COVID-19 and the flu are both spread from person-to-person, between people who are in close contact with one another (within about 6 feet). Both viruses are spread through respiratory droplets when an infected person coughs, sneezes, or talks or by touching a surface with the virus on it and then touching the eyes, nose or mouth without washing hands. Similarly, both the flu virus and the virus that causes COVID-19 may be spread to other people before symptoms develop.

COVID-19, however, is believed to be more contagious than the flu and involved in more “superspreading events” where one person infects many others. Likewise, people with COVID-19 may be contagious for longer than those with the flu. According to the CDC, most people with the flu are contagious about 1 day before showing symptoms. Older children and adults with flu appear to be most contagious during the initial 3-4 days of their illness but many remain contagious for about 7 days. Meanwhile, those with COVID-19 may be contagious for 2 days before showing symptoms and remain contagious for at least 10 days after symptoms appear or testing positive, and even longer for people with more severe cases.

High Risk

While most cases of both COVID-19 and flu are mild, both cause a variety of severe illnesses or complications. Complications from both viruses include the following:

  • Pneumonia
  • Respiratory failure
  • Acute respiratory distress syndrome
  • Sepsis
  • Cardiac injury (e.g. heart attacks and stroke)
  • Multiple-organ failure (respiratory failure, kidney failure, shock)
  • Worsening of chronic medical conditions (involving the lungs, heart, nervous system or diabetes)
  • Secondary bacterial infections (i.e. infections that occur in people who have already been infected with flu or COVID-19)
  • Inflammation of the heart, brain, or muscle tissues

Read more about distinct flu and COVID-19 complications.

While everyone is at risk of both COVID-19 and flu, adults 65 and older and people with underlying medical conditions are at higher-risk of severe illness or complications from infection. Pregnant women may also be at increased risk of illness and hospitalization from both viruses. Though children may not be at higher risk of catching either flu or COVID-19, research is showing that healthy children are at higher risk of complications from flu than COVID-19. Adolescents and teens with COVID-19 are also at higher risk of Pediatric Multisystem Inflammatory Syndrome, which is most likely rare.


The best way to protect yourself and others against both flu and COVID-19 is to get vaccinated. Multiple vaccines are produced each year to protect against the flu, while three COVID-19 vaccines have been authorized for emergency use by the FDA. These vaccines not only reduce your risk of getting the flu or COVID-19 and lessen the severity of illness if you do get sick, but they also help prevent both viruses from spreading. Getting vaccinated can also protect those around you, particularly those at increased risk for severe illness.

In addition to vaccination, many of the same health and safety precautions we’ve taken over the last year to prevent the spread of COVID-19 also help prevent the flu. The CDC recommends the following to reduce your risk of both flu and COVID-19 infection:

  • Wash your hands often with soap and water for at least 20 seconds, especially after going into public or touching common surfaces. If soap and water aren’t available, use an alcohol-based hand sanitizer.
  • Avoid touching your eyes, nose, and mouth with unwashed hands.
  • Avoid close contact with people who are sick. Close contact is considered spending more than a few minutes within 6 feet of a sick person, or sharing common surfaces or utensils.
  • Cover your cough or sneeze with a tissue, then throw the tissue in the trash.
  • Stay home when you are sick
  • Regularly clean and disinfect frequently touched objects and surfaces.

For COVID-19 specifically, the CDC also recommends practicing social distancing and wearing a face mask when sick and in certain public settings depending on your vaccination status and the level of COVID-19 transmission in your area. While these precautions would not typically be advised during a normal flu season, doing so to prevent COVID-19 will also help reduce your risk of catching the flu this year. Learn more about when to wear a face mask here.


Because it is often difficult to distinguish COVID-19 from the flu, the best way to determine your course of care is to get tested. Until you receive your results, we recommend that anyone who is experiencing the symptoms above isolate until the following, regardless of vaccination status:

  • At least 10 days have passed since their symptoms first appeared


  • They have had no fever for at least 24 hours without the use of fever-reducing medication


  • Other respiratory symptoms have improved

Read our guide on how to self-isolate.

As for flu, several prescription antiviral drugs have been approved by the FDA. Most people with the flu have mild illness and do not need medical care or antiviral drugs. In most cases you should stay home and avoid contact with other people except to get medical care. We recommend staying hydrated and taking a fever-reducing medicine like ibuprofen or acetaminophen (Tylenol) as needed. An antihistamine like Zyrtec or Claritin may help reduce your congestion regardless of whether it’s caused by a virus or allergies. For the seasonal flu, Tamiflu (oseltamivir) may be helpful for individuals at high risk of complications from the flu, but needs to be started soon after symptoms begin. It also causes fairly frequent side effects, so isn’t the best choice for everyone. Rest and hydration are the most important things to remember as you work on getting well.

Emergency Signs

The symptoms listed below should be considered warning signs, indicating medical attention should be sought right away:

  • Extreme difficulty breathing or shortness of breath
  • Persistent pain or pressure in the chest or abdomen
  • Persistent dizziness, weakness, confusion, inability to awaken
  • Seizures
  • Not urinating
  • Fever or cough that improve but then return or worsen
  • Unable to eat or drink enough
  • Worsening of chronic medical conditions

And while current research and data indicate children are least likely to experience severe symptoms or complications associated with coronavirus, the following symptoms should be addressed immediately if present in children 14 age or under:

  • Fast breathing or trouble breathing
  • Bluish lips or face
  • Ribs pulling in with each breath
  • Chest pain
  • Severe muscle pain (child refuses to walk)
  • Dehydration (no urine for 8 hours, dry mouth, no tears when crying)
  • Not alert or interacting when awake
  • Seizures
  • Fever above 104°F
  • In children less than 12 weeks, any fever
  • Fever or cough that improve but then return or worsen
  • Worsening of chronic medical conditions

If there is concern that you, or a loved one, are medically unstable as a result of the symptoms listed above, please contact 9-1-1 immediately.

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William Kimbrough, One Medical Provider
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The One Medical blog is published by One Medical, a national, modern primary care practice pairing 24/7 virtual care services with inviting and convenient in-person care at over 100 locations across the U.S. One Medical is on a mission to transform health care for all through a human-centered, technology-powered approach to caring for people at every stage of life.

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