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.
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. 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.
While more is learned every day about COVID-19 and the virus that causes it, there is still a lot that is unknown . This page compares COVID-19 and flu, given the best available information to date.
Signs and Symptoms
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
Shortness of breath or difficulty breathing
Runny or stuffy nose
Muscle pain or body aches
Some people may have vomiting and diarrhea, though this is more common in children than adults
Flu viruses can cause mild to severe illness, including common signs and symptoms listed above.
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.
How long symptoms appear after exposure and infection
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.
If a person has COVID-19, it could take them longer to develop symptoms than if they had flu.
Typically, a person develops symptoms anywhere from 1 to 4 days after infection.
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 long someone can spread the virus
For both COVID-19 and flu, it’s possible to spread the virus for at least 1 day before experiencing any symptoms.
If a person has COVID-19, they may be contagious for a longer period of time than if they had flu.
Most people with flu 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 long someone can spread the virus that causes COVID-19 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.
Both COVID-19 and flu can result in complications, including:
Acute respiratory distress syndrome (fluid in the lungs)
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)
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.
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)
People at high-risk of complications or who have been hospitalized for COVID-19 or flu should receive supportive medical care to help relieve symptoms and complications.
Prescription influenza antiviral drugs are FDA-approved to treat flu.
People who are hospitalized with flu or at high-risk of flu complications with flu symptoms are recommended to be treated with antiviral drugs as soon as possible.
While remdesivir is an antiviral agent that is being explored as a treatment for COVID-19 and is available under an Emergency Use Authorization (EUA), there are currently no drugs or other therapeutics approved by the Food and Drug Administration (FDA) to prevent or treat COVID-19. Studies are in progress to learn more.
Vaccines for COVID-19 and flu must be approved or authorized for emergency use (EUA) by the FDA.
There are multiple FDA-licensed influenza vaccines produced annually to protect against the 3 or 4 flu viruses that scientists anticipate will circulate each year.
Two COVID-19 vaccines have been approved for use by the FDA under an EUA. Other vaccines to prevent COVID-19 are still under development.