Influenza (flu), COVID-19, and respiratory syncytial virus (RSV) are all common respiratory illnesses caused by different viruses. They cause similar symptoms in many cases, but there are some key differences that set them apart.

Respiratory illnesses are common during the fall and winter months in the United States. Most people develop a fever, stuffy nose, or cough at some point during this time of year.

Read on to understand the differences between the flu, COVID-19, and RSV in their symptoms, diagnosis, and treatment.


Each year in the United States, 9 million to 41 million people get sick with the flu. While most cases are mild, up to 710,000 people become seriously ill.

Flu symptoms typically come on very suddenly. The most common symptoms include:

  • fever
  • body aches
  • chills
  • fatigue or tiredness
  • chest discomfort
  • a cough, which is typically dry
  • headache

Diarrhea can affect anyone with flu, but it’s a more common symptom in children than adults. Change in taste or smell is possible but is much less common than with COVID-19.

Although fever is common, not everyone with the flu will have one. Fevers are typically high, especially in children. Body aches and fatigue may be particularly intense.

Difficulty breathing, severe muscle or chest pain, and worsening fever or cough are some possible signs of a severe flu infection that may require emergency medical care.


People with COVID-19 can have a wide range of symptoms. They typically begin more slowly and build gradually over time.

Symptoms of COVID-19 are often similar to flu symptoms. Loss of taste or smell is more common with COVID-19, as is:

  • diarrhea
  • nausea
  • vomiting

As new coronavirus variants appear, the symptoms of COVID-19 may change.

Symptoms of severe complications from COVID-19 may include:

  • trouble breathing
  • ongoing pain or pressure in the chest
  • changes in the color of the skin, lips, or nail beds

Babies and children of all ages may require emergency care if they:

  • have a high fever of at least 104ºF (40ºC)
  • are not getting enough food or water because of symptoms


Symptoms of RSV are typically mild but may become more severe over time in vulnerable people, such as young children or older adults. Symptoms usually appear in stages, building gradually over a period of a couple of weeks.

RSV symptoms are typically cold-like and include:

  • runny nose
  • cough
  • sneezing
  • reduced appetite

In very young infants, the only symptoms may be:

  • irritability
  • lower energy levels
  • difficulty breathing

In more serious cases, RSV infections can lead to bronchiolitis or pneumonia.


In some cases, a healthcare professional may not perform any kind of testing to diagnose a person with the flu, especially if there is a high level of influenza activity in the area.

If testing is required, such as for treatment or public health tracking purposes, there are a variety of tests a doctor may use. The most commonly used tests in clinical settings are molecular and antigen tests:

  • Molecular tests: These tests detect the genetic material of the virus (influenza) that causes the flu. They can also distinguish between the different types of influenza viruses, including influenza A and B, and their different season subtypes.
  • Antigen tests: These detect specific viral proteins that are made during infection. They are less sensitive than molecular tests but can usually be done more quickly.

If antiviral treatment for the flu is needed, testing should be done within 48 hours of the start of symptoms.


Molecular and antigen tests are available for COVID-19 as well. These detect the proteins and genetic material of SARS-CoV-2, the coronavirus that causes COVID-19. Most at-home tests for COVID-19 are antigen tests.

People with symptoms of COVID-19 should get tested immediately, either at home or at a nearby clinic. After being exposed to someone with COVID-19, you should wait 5 days to test, since early testing may not provide accurate results.

Like with the flu, antiviral treatments for COVID-19 work best when started early, so testing should be performed as soon as possible.


Infections with RSV are so common, especially in children, that many people are not tested for the virus unless they are at risk of more severe disease.

If testing is needed, molecular and antigen tests are most commonly used.


Healthcare professionals may recommend antiviral treatment for some people who get sick with the flu and are more likely to develop severe illness. This includes:

  • young children
  • adults ages 65 years or older
  • pregnant people
  • people with certain medical conditions, such as asthma, diabetes, or heart disease

Antiviral treatments work best when started within 1 to 2 days of the start of symptoms. They can help make symptoms milder and shorten the time you are sick by 1 to 2 days.

There are four types of antivirals used to treat influenza in the United States:

  • oseltamivir phosphate (Tamiflu, or available as a generic)
  • zanamivir (Relenza)
  • peramivir (Rapivab)
  • baloxavir marboxil (Xoflu)

Antibiotics are not helpful for treating the flu since it’s caused by a virus, not bacteria.

Flu vaccines can help prevent illness from influenza infection or make symptoms milder if you do get sick.


People who are more likely to develop severe COVID-19 illness may be prescribed antivirals such as:

  • nirmatrelvir with ritonavir (Paxlovid)
  • remdesivir (Veklury)
  • molnupiravir (Lagevrio)

The choice of treatment will depend on your age, how recently your symptoms started, and other types of medications you may be taking.

People who have a weakened immune system and develop COVID-19 may be treated with convalescent plasma to prevent it from becoming a severe infection. This treatment is generally only given when the disease isn’t responding to other treatments.

COVID-19 vaccines can help prevent illness or make symptoms milder if you get sick.


Most RSV infections go away on their own within 1 week. You can typically manage symptoms at home with fever and pain reducers, fluids, and rest.

There are no specific treatments available for RSV. People who develop severe complications from infection may need to be hospitalized to receive oxygen and intravenous (IV) fluids. In rare cases, intubation for breathing support may be needed.

Doctors may prescribe preventive treatment with palivizumab during peak RSV seasons to young children and infants who are more likely to develop severe illness from RSV.