Flu and COVID-19 cannot be told apart by their symptoms alone, according to the Centers for Disease Control and Prevention, which says testing is needed to confirm a diagnosis. The agency says the two share many of their symptoms. The differences it describes are that a change in or loss of taste or smell is more frequent with COVID-19, and that COVID-19 symptoms can take longer to appear after infection. This page reports what the CDC states and is not personal medical advice.
The symptoms the two illnesses share
The CDC's page on the similarities and differences between flu and COVID-19 says both are contagious respiratory illnesses caused by different viruses. Flu is caused by infection with an influenza virus, and COVID-19 is caused by infection with a coronavirus, SARS-CoV-2, the page says. It lists these as common symptoms that the two share:
- Fever or feeling feverish and having chills, with a note that not everyone with flu will have a fever
- Cough
- Shortness of breath or difficulty breathing
- Fatigue (tiredness)
- Sore throat
- Runny or stuffy nose
- Muscle pain or body aches
- Headache
- Vomiting
- Diarrhea
- Change in or loss of taste or smell
Where the CDC says they differ
The differences the CDC describes are in how often a symptom occurs and when symptoms start, and it does not name a symptom that only one of the two illnesses causes.
| Question | Flu | COVID-19 |
|---|---|---|
| Change in or loss of taste or smell | Can occur | More frequent |
| Diarrhea | More frequent in children | Can occur at any age |
| When symptoms typically appear after infection | One to four days | Two to five days, and up to 14 days |
Every row comes from the CDC's flu and COVID-19 comparison page. The same page says that a person with COVID-19 could take longer from the time of infection to experience symptoms than a person with flu.
The CDC's separate symptom pages describe the start of each illness. Its page on the signs and symptoms of flu says flu symptoms usually come on suddenly. Its page on the symptoms of COVID-19 says symptoms may appear 2 to 14 days after exposure to the virus, and that they may start as mild, with some people progressing to more severe symptoms.
How each symptom list reads
The CDC keeps one symptom page for each illness, and the lists are worded slightly differently.
| CDC list for flu | CDC list for COVID-19 |
|---|---|
| Fever or feeling feverish, chills | Fever or chills |
| Cough | Cough |
| Sore throat | Sore throat |
| Runny or stuffy nose | Congestion or runny nose |
| Muscle or body aches | Muscle or body aches |
| Headaches | Headache |
| Fatigue (tiredness) | Fatigue |
| Vomiting and diarrhea in some people | Nausea or vomiting, and diarrhea |
| Not on the list | Shortness of breath or difficulty breathing |
| Not on the list | New loss of taste or smell |
The left column comes from the CDC's flu symptoms page, which says vomiting and diarrhea are more common in children than adults, and the right column comes from its COVID-19 symptoms page. For more on the flu list, see What are the symptoms of the flu?
Testing and having both at once
The CDC's comparison page says testing can also show whether someone has both flu and COVID-19 at the same time, which it describes as uncommon. The page says people with both at the same time can have more severe illness than people with either one alone.
How long each one spreads
The CDC says a person with COVID-19 could be contagious for a longer time than a person with flu.
| Question | Flu | COVID-19 |
|---|---|---|
| Spread before symptoms | About one day before | Two to three days before, on average |
| Period the CDC highlights | Most contagious in the first three days of illness, for older children and adults | Contagious for about eight days after symptoms began, on average |
The page says young children and people with weakened immune systems may be contagious with flu for longer periods of time. For how long flu illness lasts, see How long does the flu last?
Emergency warning signs the CDC lists for COVID-19
The CDC's COVID-19 symptoms page lists these as emergency warning signs for COVID-19:
- Trouble breathing
- Persistent pain or pressure in the chest
- New confusion
- Inability to wake or stay awake
- Lips, nail beds and skin that may appear pale, gray or blue, depending on skin tone
The CDC's emergency warning signs for flu in children and adults are listed on What are the symptoms of the flu?
For how much flu and other respiratory illness is circulating now, see Is flu and respiratory illness high in my state? For the months when flu spreads most, see When is flu season?