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 coronavirus first identified in 2019. Flu is caused by infection with a flu virus (influenza viruses).
From what we know, COVID-19 spreads more easily than flu. Efforts to maximize the proportion of people in the United States who are up to date with their COVID-19 vaccines remain critical to ending the COVID-19 pandemic. More information is available about COVID-19 vaccines and how well they work.
Compared to flu, COVID-19 can cause more serious illnesses in some people. COVID-19 can also take longer before people show symptoms, and people can remain contagious for longer periods of time. More information about differences between flu and COVID-19 is available in the different sections below.
You cannot tell the difference between flu and COVID-19 just by looking at the symptoms alone because they have some of the same symptoms. That’s why testing is needed to tell what the illness is and to confirm a diagnosis. Testing is also important because it can reveal if someone has both the flu and COVID-19 at the same time.
While more is learned every day about COVID-19 and the virus that causes it, there are still things, such as post-COVID conditions (also known as long COVID), that are 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 symptoms, ranging from no symptoms (asymptomatic) to severe symptoms. Common symptoms that COVID-19 and flu share include:
- Fever or feeling feverish/having chills
- Shortness of breath or difficulty breathing
- Fatigue (tiredness)
- Sore throat
- Runny or stuffy nose
- Muscle pain or body aches
- Change in or loss of taste or smell, although this is more frequent with COVID-19.
How Long Symptoms Appear After Exposure and Infection
For both COVID-19 and flu, 1 or more days can pass from when a person becomes infected to when they start to experience symptoms of illness.
If a person has COVID-19, it could take them longer to experience symptoms than if they have flu.
Typically, a person may experience symptoms anywhere from 1 to 4 days after infection.
Typically, a person may experience symptoms anywhere from 2 to 14 days after infection.
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 could be contagious for a longer time than if they have 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 first 3-4 days of their illness, but many people 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 being studied.
Here’s what we know from studies of prior variants, including Delta:
- On average, people can begin spreading the virus 2-3 days before their symptoms begin, but infectiousness peaks 1 day before their symptoms begin.
- On average, people can continue to spread the virus another 8 days after their symptoms began.
This information is from studies of prior variants. We still have much to learn about new variants, including Omicron. CDC will share findings from ongoing studies as soon as they are available.
How it Spreads
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 large and small particles containing virus that are expelled when people with the illness (COVID-19 or flu) cough, sneeze, or talk. These particles can land in the mouths or noses of people who are nearby and possibly be inhaled into the lungs. In some circumstances, such as indoor settings with poor ventilation, small particles might be spread further than 6 feet and cause infections.
Although most spread is by inhalation, it may be possible that a person can get infected by touching another person (for example, shaking hands with someone who has the virus on their hands), or by touching a surface or object that has virus on it, and then touching their own mouth, nose, or eyes.
Both flu viruses and the virus that causes COVID-19 can be spread to others by people before they begin showing symptoms; by people with very mild symptoms; and by people who never experience symptoms (asymptomatic people).
While the virus that causes COVID-19 and flu viruses are thought to spread in similar ways, the virus that causes COVID-19 is generally more contagious than flu viruses. 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 continual spreading among people as time progresses.
People at Higher Risk for Severe Illness
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 (including infants and children)
- Pregnant people
Overall, COVID-19 seems to cause more serious illnesses in some people.
Serious COVID-19 illness resulting in hospitalization and death can occur even in healthy people.
Some people that had COVID-19 can go on to develop post-COVID conditions or multisystem inflammatory syndrome (MIS)
Both COVID-19 and flu can result in complications, including:
- Respiratory failure
- Acute respiratory distress syndrome (fluid in the lungs)
- Sepsis (a life-threatening illness caused by the body’s extreme response to an infection)
- 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 infections (bacterial or fungal infections that can occur in people who have already been infected with flu or COVID-19)
Most people who get flu will recover on their own in a few days to two weeks, but some people will experience severe complications, requiring hospitalization. Some of these complications are listed above. Secondary bacterial infections are more common with influenza than with COVID-19.
Diarrhea is more common in young children with flu than in adults with flu.
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) and in Adults (MIS-A)
Post-COVID Conditions are a range of symptoms that can last weeks or months after first being infected with the virus that causes COVID-19 or can appear weeks after infection. Long COVID can happen to anyone who has had COVID-19, even if their illness was mild, or if they had no symptoms.
People at higher 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 who are at increased risk of complications and have flu symptoms are recommended to be treated with antiviral drugs as soon as possible after illness onset.
The National Institutes of Health (NIH) has developed guidance on treatment of COVID-19, which is regularly updated as new evidence on treatment options emerge.
The Food and Drug Administration (FDA) has approved one drug, remdesivir (Veklury®), to treat COVID-19. FDA has issued emergency use authorization (EUAs) to allow healthcare providers to use investigational products that are not yet approved, or that are approved for other uses, to treat patients with COVID-19 if certain legal requirements are met.
Vaccines for COVID-19 and flu are approved and/or authorized for emergency use (EUA) by FDA.
There are multiple FDA-licensed influenza vaccines produced annually to protect against the 4 flu viruses that scientists expect will circulate each year.
Three COVID-19 vaccines are authorized or approved for use in the United States to help prevent COVID-19. Pfizer-BioNTech or Moderna (COVID-19 mRNA vaccines) are preferred. You may get Johnson & Johnson’s Janssen COVID-19 vaccine in some situations. Other vaccines to prevent COVID-19 are under development.
Page last reviewed: January 18, 2022
Content source: Centers for Disease Control and Prevention, National Center for Immunization and Respiratory Diseases (NCIRD)
Original Article – https://www.cdc.gov/flu/symptoms/flu-vs-covid19.htm