Influenza (flu) can cause mild to severe illness, and at times can lead to death. Flu is different from a cold. Flu usually comes on suddenly. People who have flu often feel some or all of these symptoms:

• fever* or feeling feverish/chills
• cough
• sore throat
• runny or stuffy nose
• muscle or body aches
• headaches
• fatigue (tiredness)
• some people may have vomiting and diarrhea, though this is more common in children than adults.

*It’s important to note that not everyone with flu will have a fever.

Most experts believe that flu viruses spread mainly by tiny droplets made when people with flu cough, sneeze or talk. These droplets can land in the mouths or noses of people who are nearby. Less often, a person might get flu by touching a surface or object that has flu virus on it and then touching their own mouth, nose or possibly their eyes.

Following is a list of all the health and age factors that are known to increase a person’s risk of getting serious complications from flu:

• Adults 65 years and older
• Children younger than 2 years old1
• Asthma
• Neurologic and neurodevelopment conditions
• Blood disorders (such as sickle cell disease)
• Chronic lung disease (such as chronic obstructive pulmonary disease [COPD] and cystic fibrosis)
• Endocrine disorders (such as diabetes mellitus)
• Heart disease (such as congenital heart disease, congestive heart failure and coronary artery disease)
• Kidney diseases
• Liver disorders
• Metabolic disorders (such as inherited metabolic disorders and mitochondrial disorders)
• People who are obese with a body mass index [BMI] of 40 or higher
• People younger than 19 years old on long-term aspirin- or salicylate-containing medications.
• People with a weakened immune system due to disease (such as people with HIV or AIDS, or some cancers such as leukemia) or medications (such as those receiving chemotherapy or radiation treatment for cancer, or persons with chronic conditions requiring chronic corticosteroids or other drugs that suppress the immune system)
• People who have had a stroke

Other people at high risk from the flu:

• Pregnant women and women up to 2 weeks after the end of pregnancy
• American Indians and Alaska Natives
• People who live in nursing homes and other long-term care facilities
• 1 Although all children younger than 5 years old are considered at high risk for serious flu complications, the highest risk is for those younger than 2 years old, with the highest hospitalization and death rates among infants younger than 6 months old.

Information on groups at higher risk from COVID-19 is available.

Flu shots cause antibodies to develop in the body about two weeks after vaccination. These antibodies provide protection against infection with the viruses that are used to make the shot.

The seasonal flu shot protects against the influenza viruses that research indicates will be most common during the upcoming season.

Most flu shots in the United States protect against four different flu viruses (“quadrivalent”); an influenza A (H1N1) virus, an influenza A (H3N2) virus, and two influenza B viruses. There are also some flu vaccines that protect against three different flu viruses (“trivalent”); an influenza A (H1N1) virus, an influenza A (H3N2) virus, and one influenza B virus. Two of the trivalent shots are designed specifically for people 65 and older to create a stronger immune response.

If you got the flu shot last flu season (or any other past flu season), you still need a flu shot this season. A person's immune protection from vaccination declines over time and the flu viruses are constantly changing. Flu shots are updated each season to protect against the virus that may be the most common during the current flu season.

Flu shots have a great safety record. Hundreds of millions of Americans have safely received flu shots over the last 50 years. A flu shots is the first and best way to reduce your chances of getting the flu and spreading it to others.

Everyone six months of age and older should get a flu shot every season, with only a few rare exceptions.

People who should NOT get the flu shot include:

• Children younger than six months of age
• People with severe, life-threatening allergies to the flu shot or any ingredient in the shot

Similariites:

Both COVID-19 and flu can have varying degrees of signs and symptoms, ranging from no symptoms (asymptomatic) to severe symptoms. Common symptoms that COVID-19 and flu share include:

• Fever or feeling feverish/chills
• Cough
• Shortness of breath or difficulty breathing
• Fatigue (tiredness)
• Sore throat
• Runny or stuffy nose
• Muscle pain or body aches
• Headache
• Some people may have vomiting and diarrhea, though this is more common in children than adults

For both COVID-19 and flu, 1 or more days can pass between a person becoming infected and when he or she starts to experience illness symptoms.

Differences:

Flu viruses can cause mild to severe illness, including common signs and symptoms listed above (Flu Symptoms). Other signs and symptoms of COVID-19, different from flu, may include change in or loss of taste or smell (COVID-19 Symptoms)

If a person has COVID-19, it could take them longer to develop symptoms than if they had flu. Typically, a person develops flu symptoms anywhere from 1 to 4 days after infection. Whereas a person typically develops COVID-19 symptoms 5 days after being infected, but symptoms can appear as early as 2 days after infection or as late as 14 days after infection, and the time range can vary.

While COVID-19 and flu viruses are thought to spread in similar ways, COVID-19 is more contagious among certain populations and age groups than flu. Also, COVID-19 has been observed to have more superspreading events than flu. This means the virus that causes COVID-19 can quickly and easily spread to a lot of people and result in continuous spreading among people as time progresses.

A flu shot WILL NOT protect you against COVID-19. However the flu shot has been shown to reduce the risk of flu illness, hospitalization and death. Getting a flu shot this fall is more important than ever to reduce your risk from flu.