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.

Flu symptoms

Each year in the United States, 9.3 million to 41 millionTrusted Source 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. SymptomsTrusted Source may include:

  • fever
  • body aches
  • chills
  • fatigue or tiredness
  • headache
  • vomiting and diarrhea (more common in children)

Diarrhea can affect anyone with flu, but it’s a more common symptom in children than adults.

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 signsTrusted Source of a severe flu infection that may require emergency medical care.

COVID-19 symptoms

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

Symptoms of COVID-19Trusted Source are often similar to flu symptoms. Loss of taste or smell is more common with COVID-19. COVID-19 symptoms may include:

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

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 respiratory distress
  • signs of dehydration:
    • a dry mouth
    • few or no tears when crying
    • sunken eyes
    • the soft spot (fontanelle) on top of the head looks sunken (babies)
    • fewer wet diapers
    • fussy
    • drowsy
    • high fever
    • not eating enough
    • not drinking enough

RSV 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 4 or 5 days.

RSV symptomsTrusted Source are typically cold-like and include:

  • coughing
  • decrease in appetite
  • fever
  • runny nose
  • sneezing
  • wheezing

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.

Diagnosing the Flu

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 commonlyTrusted Source 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. Treatment should begin within 48 hoursTrusted Source of the start of symptoms.

Diagnosing COVID-19

Molecular and antigen tests are also available for COVID-19. 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-19Trusted Source work best when started early, so testing should be performed as soon as possible.

Diagnosing RSV

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.

Antiviral medications are typically not prescribed for RSV.

Flu treatment

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

  • young children
  • adults age 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 typesTrusted Source of antivirals used to treat influenza in the United States:

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

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. Discuss getting a flu vaccineTrusted Source for you or your child with your doctor or healthcare professional.

COVID-19 treatment

People who are more likely to develop severe COVID-19 illness may be prescribed antiviralsTrusted Source 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.

COVID-19 vaccinesTrusted Source can help prevent illness or make symptoms milder if you get sick. The CDC recommends the 2023–2024 updated COVID-19 vaccines: Pfizer-BioNTech, Moderna, or Novavax, to protect against serious illness from COVID-19. Specific recommendations are based on your age, time since your last dose, the first vaccine received, and immunocompromised status. Speak with your doctor of healthcare professional about a COVID-19 vaccine

RSV treatment

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.

The CDC recommendsTrusted Source the following to help prevent RSV disease in high risk groups:

  • Adults 60 years of age and older: Receive a single dose of RSV vaccine.
  • Pregnant people: A single dose of the RSV vaccine is recommended for pregnant people from week 32 through week 36 of pregnancy to help prevent RSV disease in infants under 6 months of age. This vaccine is recommended to be given from September through January for most of the United States.
  • Infants and young children: Infants younger than 8 months of age entering their first RSV season (typically fall through spring) and those ages 8 to 19 months at an increased risk of severe RSV and entering their second RSV season are recommended to receive a dose of RSV preventive antibody nirsevimab.

How can you tell the difference between the flu, COVID-19, and RSV?

Since the symptoms can be very similarTrusted Source, a lab test may be the only way to know exactly which virus you have.

How long do the symptoms of the Flu, COVID-19, and RSV last?

For most people, the flu may lastTrusted Source from a few days to 2 weeks, COVID-19 may lastTrusted Source between 1 and 14 days, and RSV symptoms can last between 1 and 2 weeks. Some individuals are at higher risk from these viruses and may develop serious complications, require hospitalization, or experience life threatening illness.

The flu, COVID-19, and RSV are common respiratory illnesses caused by different viruses. The symptoms may be similar, but there are often differences that help you and healthcare professionals determine which one you have.

Many people recover from these viruses without medical treatment, but if necessary, there are lab tests that can identify the exact virus you have and may help plan treatment and prevent serious complications.