Flu vs. COVID-19: How To Tell The Difference Between Each Virus

The flu is so well-known that the term “flu-like symptoms” is commonly used as a way to describe the effects of other diseases—including COVID-19. Because its symptoms can be similar to those of the flu, COVID is sometimes compared to the flu. But medical professionals stress that the two are different in important ways. And knowing the difference is crucial to understanding the risks of each illness.

In this article, we’ll compare the flu and COVID-19 and break down their differences by definition. For expert medical information, read the detailed comparison by the Centers for Disease Control and Prevention (CDC).

Quick summary

COVID-19 is not the same as the flu, nor is it a version of the flu. The flu and COVID-19 are both viruses, but they are different viruses. They sometimes have similar symptoms, and both have the potential to be severe, but COVID-19 is much more likely to cause severe health problems, including severe respiratory issues and death, especially in certain vulnerable populations.

What is the flu?

The word flu in the name of the illness that we call “the flu” is short for influenza, a contagious disease caused by a virus. It’s not just a single virus that causes the flu—influenza is caused by a large family of influenza viruses. There are four major types and many different subtypes.

The flu is highly contagious and typically spreads via transfer of bodily fluids, such as mucus or saliva. Typically, fluids containing the flu virus travel through the air in the form of very small droplets (released when an infected person sneezes, for example) that can enter another person’s body, often into their respiratory system through their nose or mouth.

The flu is considered endemic in many places, meaning that it persistently and regularly spreads within the region (that is, it never fully goes away). In the US, flu activity increases every year during the fall and winter months starting around October. This time of increased flu activity is commonly called flu season. The severity of flu season often depends on which specific influenza viruses are circulating at that time. Extreme flu outbreaks can and have resulted in flu epidemics.

What are symptoms of the flu?

Some common symptoms of the flu include:

  • fever or chills
  • cough
  • sore throat
  • runny/stuffy nose
  • muscle aches
  • headaches
  • fatigue
  • vomiting or diarrhea (more common in children)

Many people experience the flu as being like a very bad cold with the additional symptoms of achiness and fatigue, among others. Most cases of the flu resolve without medical intervention, but severe cases do occur, and can be especially dangerous in vulnerable populations, particularly young children, older people, and people with compromised immune systems.

Supporting one’s immune system can mean many things, from vaccinating to inoculating to immunizing. Learn what each one means.

What type of virus is COVID-19?

COVID-19 is short for Coronavirus Disease 2019, a specific strain (SARS-CoV-2) of the coronavirus that emerged in 2019 and spread rapidly around the world in 2020, resulting in a global pandemic. Coronaviruses are a large family of viruses that typically cause respiratory illness. Most coronaviruses result in sicknesses of mild or medium severity.

COVID-19 is a potentially severe respiratory illness—it affects the lungs and can result in difficulty breathing, among more severe problems. Spread of COVID-19 is typically the result of airborne transfer—meaning through droplets in the air (such as from coughs or sneezes by infected people).

COVID-19 is extremely contagious and spreads very quickly. There are several mutated variants of the virus, and some are even more contagious.

What are symptoms of COVID-19?

Common symptoms of COVID-19 include:

  • fever or chills
  • cough
  • shortness of breath or difficulty breathing
  • fatigue
  • muscle or body aches
  • headache
  • loss of taste or smell
  • sore throat
  • congestion or runny nose
  • nausea or vomiting
  • diarrhea

The severity of COVID-19 varies widely. Some people may experience mild symptoms or even be completely asymptomatic (meaning they have no symptoms at all), while others may experience severe respiratory illness, including cases that result in major organ damage or death. Long-term effects are still being studied.

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What’s the difference between the flu and COVID-19?

The flu and COVID-19 are both viruses, but they are different viruses: the flu is caused by an influenza virus and COVID-19 is caused by a coronavirus. They sometimes have similar symptoms, but they affect the body in different ways.

Although both illnesses can potentially be severe, data has shown that COVID-19 is more likely to cause severe respiratory issues and some other severe health problems. Because COVID-19 is relatively new, its potential long-term effects are not fully known, and treatment methods are still being developed.

Data also shows that certain populations of people, such as older adults, are much more likely to experience severe issues when infected with COVID-19 than with the flu.

A person infected with COVID-19 is also more likely to be considered contagious for a longer span of time than someone infected with the flu.

How do viruses differ from bacteria? Read up on their differences.

Vaccines are available for both COVID-19 and the flu. Just as the flu and COVID-19 are different illnesses, the vaccines are different. A flu vaccine will NOT protect someone against COVID-19, nor will a COVID-19 vaccine protect someone against the flu. (This is because vaccines are designed based on the specific disease they are intended to prevent.)

Health experts generally recommend that most people get vaccinated for both COVID-19 and the flu. From a public health standpoint, this is especially important due to the fact that the spread of both can happen simultaneously. (This dual-outbreak scenario is sometimes referred to as a twindemic. In contrast, the informal term flurona refers to a case of an individual person being simultaneously infected with both the flu and the COVID-19 virus.)

Getting vaccinated helps to protect not only the people who receive the vaccine, but also those around them. This in turn also helps to lessen the strain on hospitals and healthcare facilities experiencing a high rate of patients due to a sustained amount of COVID-19 cases.

When it comes to arming yourself against diseases, terms like "efficiency," "effectiveness," and "efficacy" can come up. Understand their differences here.

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