Understanding the Differences

What are the differences between Influenza (Flu) and COVID-19? The CDC explains.

Influenza (Flu) and COVID-19 are both contagious respiratory illnesses, but they are caused by different viruses, according to the CDC.

“COVID-19 is caused by infection with a new coronavirus (called SARS-CoV-2) and flu is caused by infection with influenza viruses,” the CDC said on its website. “Because some of the symptoms of flu and COVID-19 are similar, it may be hard to tell the difference between them based on symptoms alone, and testing may be needed to help confirm a diagnosis. Flu and COVID-19 share many characteristics, but there are some key differences between the two.

While more is learned every day, there is still a lot that is unknown about COVID-19 and the virus that causes it. The following table compares COVID-19 and flu, given the best available information to date.

Signs and symptoms

Similarities

Both COVID-19 and flu can have varying degrees of signs and symptoms, ranging from no symptoms (asymptomatic) to severe symptoms. Common symptoms of COVID-19 and flu include:

  • Fever or feeling feverish/chills
  • Cough
  • Shortness of breath or difficulty breathing
  • Fatigue (tiredness)
  • Sore throat
  • Runny or stuffy nose
  • Muscle pain or body aches
  • Headache
  • Some people may have vomiting and diarrhea, though this is more common in children than adults

Differences

Other signs and symptoms of COVID-19 may include change in or loss of taste or smell. (More symptoms may be added following this issue.)

How long symptoms appear after exposure and infection

Similarities

For both COVID-19 and flu, 1 or more days can pass between a person becoming infected and when he or she starts to experience illness symptoms.       

Differences

If a person has COVID-19, it could take them longer to develop symptoms than if they had flu.

COVID-19

  • Typically, a person develops symptoms 5 days after being infected, but symptoms can appear as early as 2 days after infection or as late as 14 days after infection, and the time range can vary.

Flu

  • Typically, a person develops symptoms anywhere from 1 to 4 days after infection.

How long someone can spread the virus

Similarities

For both COVID-19 and flu, it’s possible to spread the virus for at least 1 day before experiencing any symptoms.   

Differences

If a person has COVID-19, they may be contagious for a longer period of time than if they had flu.

COVID-19

  • How long someone can spread the virus is still under investigation. It’s possible for people to spread the virus for about 2 days before experiencing signs or symptoms and remain contagious for at least 10 days after signs or symptoms first appeared.
  • If someone is asymptomatic or their symptoms go away, it’s possible to remain contagious for at least 10 days after testing positive for COVID-19.

Flu

  • Most people are contagious for about 1 day before they show 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. Infants and people with weakened immune systems can be contagious for even longer.

How it spreads

Similarities

  • Both COVID-19 and flu can spread from person-to-person, between people who are in close contact with one another (within about 6 feet).
  • Both are spread mainly by droplets made when people with the illness (COVID-19 or flu) cough, sneeze, or talk.
  • These droplets can land in the mouths or noses of people who are nearby or possibly be inhaled into the lungs.
  • It may be possible that a person can get infected by physical human contact (e.g. shaking hands) or by touching a surface or object that has virus on it and then touching his or her own mouth, nose, or possibly their eyes.
  • Both flu virus and SARS-CoV-2 may be spread to others by people before they begin showing symptoms, with very mild symptoms or who never developed symptoms (asymptomatic).

Differences

While COVID-19 and flu viruses are thought to spread in similar ways, COVID-19 is more contagious among certain populations and age groups than flu. Also, COVID-19 has been observed to have more superspreading events than flu. This means the virus that causes COVID-19 can quickly and easily spread to a lot of people and result in continuous spreading among people as time progresses.

People at high-risk for severe illness

Similarities

Both COVID-19 and flu illness can result in severe illness and complications. Those at highest risk include:

  • Older adults
  • People with certain underlying medical conditions
  • Pregnant people

Differences

COVID-19

  • School-aged children infected with COVID-19 are at higher risk of Multisystem Inflammatory Syndrome in Children (MIS-C), a rare but severe complication of COVID-19.

Flu

  • Young children*

* The risk of complications for healthy children is higher for flu compared to COVID-19. However, infants and children with underlying medical conditions are at increased risk for both flu and COVID-19.

Complications

Similarities

Both COVID-19 and flu can result in complications, including:

  • Pneumonia
  • Respiratory failure
  • Acute respiratory distress syndrome (i.e. fluid in lungs)
  • 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)
  • Inflammation of the heart, brain or muscle tissues
  • Secondary bacterial infections (i.e. infections that occur in people who have already been infected with flu or COVID-19)

Differences

Additional complications associated with COVID-19 can include:

  • Blood clots in the veins and arteries of the lungs, heart, legs or brain Multisystem Inflammatory Syndrome in Children (MIS-C)