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Photo by engin akyurt on Unsplash (rights-free)

By: Gordon Chan

Covid-19 and the seasonal flu are both infectious respiratory illnesses, yet they are caused by very different viruses. Covid-19 is caused by infection with the virus Sars-CoV-2, whereas the flu is caused by infection with the influenza virus. This fall and winter season is the first time we will experience the flu season wrapped in a global pandemic. Distinguishing between the flu and Covid-19 will have important prognostic implications, so it’s imperative that we all learn the differences. Mistakenly treating patients with influenza as though they have Covid-19 can be extremely wasteful and potentially harmful, and vice versa. Healthcare providers should aim to test for both illnesses before treating for one virus over the other. 

Signs and Symptoms

Some symptoms of flu and Covid-19 are similar, so it may be hard to differentiate between them based on symptoms alone. Common symptoms that Covid-19 and flu share include:

  • Fever or feeling feverish 
  • Shortness of breath or trouble breathing
  • Sore throat
  • Runny or stuffy nose
  • Muscle pain or body aches
  • Headache
  • Fatigue or tiredness
  • Cough  

There are some key differences between the flu and Covid-19. The main one being that Covid-19 seems to have more long term consequences. People with Covid-19 have reported varying degrees of symptoms, ranging from completely asymptomatic to severely debilitating. Some unusual symptoms caused by Covid-19 include:

  • Loss of taste or smell
  • Gastrointestinal symptoms (vomiting, diarrhea) 
  • Conjunctivitis (pink eye)

Incubation period

Covid-19 symptoms typically develop 5 days after the initial infection; however, symptoms have been reported to appear as early as 2 days or as late as 14 days after an infection. On the other hand, a typical person with the flu develops symptoms 1-4 days after infection.

For both Covid-19 and flu, infected individuals can spread the virus for at least 1 day before they experience any symptoms. However, the main difference is that people with Covid-19 may be contagious for a longer period of time than if they had the flu. If someone is asymptomatic or their symptoms go away, they may not realize they are contagious and are more likely to infect others. It’s possible to remain contagious for at least 10 days after testing positive. 

Because Sars-CoV-2 is a new virus, our bodies are unlikely to have any existing antibodies. Most of us are carrying some level of antibody protection from previous exposure to the flu or from flu shots. Therefore, the Covid-19 virus seems to be spreading much faster than the flu.

High Risk Individuals

The risk of developing severe symptoms of Covid-19 increases with age. Other high risk individuals include pregnant women and individuals with pre-existing conditions. Pregnant women are also at high risk for developing severe complications. These conditions include, but are not limited to:

  • Lung problems (cystic fibrosis, asthma)
  • Heart disease (coronary artery disease, cardiomyopathy, previous heart failure) 
  • Diabetes and obesity 
  • Blood disorders (sickle cell anemia, thalassemia) 
  • Weakened immune system (cancer treatments, organ transplant, HIV/AIDS)
  • Chronic kidney or liver disease


Both Covid-19 and flu can cause a wide range  of complications, including:

  • Pneumonia
  • Respiratory failure
  • Sepsis
  • Cardiac problems (heart attack, stroke) 
  • Multiple organ failure
  • Worsening of chronic medical conditions
  • Inflammation 

Most people who get the flu usually recover within a couple days to less than two weeks. However, Covid-19 survivors report more long-term effects of the infection than influenza survivors. Lingering Symptoms of Covid-19 include:

  • Long standing organ damage (brain, great, lungs) 
  • Blood clots and weakened blood vessels 
  • PTSD, depression, and anxiety
  • Chronic fatigue syndrome
  • Severe acid reflux and gastrointestinal damage

Approved treatments

One of the reasons why Covid-19 is so dangerous is because almost everyone is susceptible to infection. Unlike the flu, which many people have immunity to, few people would have pre-existing immunity to Covid-19 because it is a relatively new virus. 

Most people with the flu have mild symptoms and do not need to be hospitalized. Getting a flu vaccine is the best way to prevent seasonal flu and potentially serious symptoms. In the case you get sick, there are antiviral drugs to treat the flu. These antiviral drugs can make illness milder and shorten the time you are sick, while also preventing more serious flu complications. 

On the other hand, Covid-19 currently has few known treatments. While blood thinners and antiviral antibodies might be effective at treating patients with Covid-19, expectations have been relatively low. 

Many companies are now in the race to create the first vaccine; however, only remdesivir has been approved to treat Covid-19. Remdesivir is an antiviral medication that was developed by Gilead Sciences. The drug mimics molecules the virus needs to replicate so the virus can’t reproduce as much within the patient’s body. Studies have demonstrated that remdesivir is most effective early on during the progression of Covid-19 and becomes less effective in later stages.

Can you have Covid-19 and flu at the same time?

Yes, while extremely rare, it is possible to be infected with both viruses at the same time. A study published in the Cureus Journal of Medical Science found that 3% of patients hospitalized with Covid-19 were also co-infected with other respiratory diseases, such as the flu. 

Since many symptoms of Covid-19 and flu overlap, it can be extremely difficult to tell the difference between them based on symptoms alone. The best way to determine if you are sick with flu or Covid-19 is to take a diagnostic test. The CDC has developed a test that can check for seasonal flu viruses and Sars-CoV-2. The Influenza Sars-Cov-2 (Flu SC2) Multiplex Assay provides sensitive and accurate results while saving time and testing materials. The diagnostic test was granted emergency use authorization back in early July. 

What to do if you feel sick?

If you are feeling sick and unsure whether you have the flu or Covid-19, you should quickly contact your healthcare provider. Tell them you may have Covid-19 so they can take the necessary precautions to protect themselves and other patients. Most importantly, you should always wear a mask over your nose and mouth whenever you’re around others.

For additional information, the CDC continually updates their guidelines about what to do if you are sick