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 difference 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 no vaccine to prevent COVID-19. The best way to prevent infection is to avoid being exposed to the virus.
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 abut COVID-19 and the virus that causes it. 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. Not everyone with flu will have a fever. 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. 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.
Anyone can get sick with flu (even healthy people), and serious problems related to flu can happen at any age, but some people are at high risk of developing serious flu-related complications if they get sick. This includes people 5 years and older, people of any age with certain chronic medical conditions (such as asthma, diabetes, or heart disease), pregnant women and children younger than 5 years, but especially those younger than 2 years old.
COVID-19 seems to cause more serious illnesses in some people. Other signs and symptoms of COVID-19, different form flu, may include change in or loss of taste or smell. Anyone can have mild to severe symptoms. Older adults and people who have severe underlying medical conditions like heart or lung disease or diabetes seem to be at higher risk for developing more serious complications from COVID-19 illness. 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.
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 for the flu, a person develops symptoms anywhere from 1-4 days after infection. With COVID-19, 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 can vary.
For 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. 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 spread from person-to-person, between people who are in close contact with one another (within 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 (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 the virus that causes COVID-19 may be spread to others by people before they begin showing symptoms, with very mild symptoms or who never developed symptoms (asymptomatic).
While COVID-19 and flu viruses are though 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 that 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.