What is the difference between Influenza (Flu) and COVID-19?
Influenza (Flu) and COVID-19 are both contagious respiratory illnesses, but they are caused by different viruses. COVID-19 is caused by infection with a new coronavirus (called SARS-CoV-2) and flu is caused by infection with influenza viruses.
There are some key differences between flu and COVID-19. COVID-19 seems to spread more easily than flu and causes more serious illnesses in some people. It can also take longer before people show symptoms and people can be contagious for longer. Another important difference is there is a vaccine to protect against flu. There is currently a limited supply of COVID-19 vaccine in the United States, but supply will increase in the weeks and months to come. The best way to prevent infection is to avoid being exposed to the virus. More information about differences between flu and COVID-19 is available in the different sections below.
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.
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 that COVID-19 and flu share 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:
Flu
Flu viruses can cause mild to severe illness, including common signs and symptoms listed above.
Flu Symptoms
Influenza (flu) can cause mild to severe illness, and at times can lead to death. Flu is different from a cold. Flu usually comes on suddenly. People who have flu often feel some or all of these symptoms:
- fever* or feeling feverish/chills
- cough
- sore throat
- runny or stuffy nose
- muscle or body aches
- headaches
- fatigue (tiredness)
- some people may have vomiting and diarrhea, though this is more common in children than adults.
*It’s important to note that not everyone with flu will have a fever.
COVID-19
COVID-19 seems to cause more serious illnesses in some people. Other signs and symptoms of COVID-19, different from flu, may include change in or loss of taste or smell.
COVID-19 Symptoms
People with COVID-19 have had a wide range of symptoms reported – ranging from mild symptoms to severe illness. Symptoms may appear 2-14 days after exposure to the virus. People with these symptoms may have COVID-19:
- Fever or chills
- Cough
- Shortness of breath or difficulty breathing
- Fatigue
- Muscle or body aches
- Headache
- New loss of taste or smell
- Sore throat
- Congestion or runny nose
- Nausea or vomiting
- Diarrhea
This list does not include all possible symptoms. CDC will continue to update this list as we learn more about COVID-19.
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.
Flu
Typically, a person develops symptoms anywhere from 1 to 4 days after infection.
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.
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 (for example, shaking hands) or by touching a surface or object that has virus on it and then touching their own mouth, nose, or possibly their eyes.
Both flu virus and the virus that causes COVID-19 may be spread to others by people before they begin showing symptoms; by people with very mild symptoms; and by people 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:
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.
Flu
Young children are at higher risk of severe illness from flu.
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.
Complications
Similarities:
Both COVID-19 and flu can result in complications, including:
- Pneumonia
- Respiratory failure
- Acute respiratory distress syndrome (fluid in the lungs)
- Sepsis
- Cardiac injury (for example, heart attacks and stroke)
- Multiple-organ failure (respiratory failure, kidney failure, shock)
- Worsening of chronic medical conditions (involving the lungs, heart, or nervous system or diabetes)
- Inflammation of the heart, brain, or muscle tissues
- Secondary bacterial infections (infections that occur in people who have already been infected with flu or COVID-19)
Differences:
Flu
Most people who get flu will recover in a few days to less than two weeks, but some people will develop complications. Some of these complications are listed above.
Flu complications:
Most people who get flu will recover in a few days to less than two weeks, but some people will develop complications (such as pneumonia) as a result of flu, some of which can be life-threatening and result in death.
Sinus and ear infections are examples of moderate complications from flu, while pneumonia is a serious flu complication that can result from either influenza virus infection alone or from co-infection of flu virus and bacteria. Other possible serious complications triggered by flu can include inflammation of the heart (myocarditis), brain (encephalitis) or muscle (myositis, rhabdomyolysis) tissues, and multi-organ failure (for example, respiratory and kidney failure). Flu virus infection of the respiratory tract can trigger an extreme inflammatory response in the body and can lead to sepsis, the body’s life-threatening response to infection. Flu also can make chronic medical problems worse. For example, people with asthma may experience asthma attacks while they have flu, and people with chronic heart disease may experience a worsening of this condition triggered by flu.
COVID-19
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) is a condition where different body parts can become inflamed, including the heart, lungs, kidneys, brain, skin, eyes, or gastrointestinal organs. We do not yet know what causes MIS-C. However, we know that many children with MIS-C had the virus that causes COVID-19, or had been around someone with COVID-19. MIS-C can be serious, even deadly, but most children who were diagnosed with this condition have gotten better with medical care.