Influenza

Overview

Influenza, also known as the flu, is a highly contagious respiratory illness caused by influenza viruses that infect the nose, throat, and lungs. The two main types of influenza viruses that typically spread among people and cause annual flu outbreaks are types A and B.

The flu is a mild illness for some people. Most people who develop the flu feel better after a few days to two weeks. For others, such as older adults, young children, and those with chronic health conditions, it can be very serious and even life-threatening.

An influenza pandemic occurs when a new influenza virus emerges that can infect and spread easily from person to person, and to which most of the global population has little or no immunity. The last known influenza pandemic occurred in 2009 with the emergence of the H1N1 influenza virus.

Signs and Symptoms

People with the flu can experience a range of symptoms. Common flu symptoms include:

• Fever and/or chills
• Headache
• Muscle pain or body aches
• Feeling tired or weak
• Dry cough
• Sore throat
• Runny or stuffy nose
• Sneezing
• Shortness of breath or difficulty breathing
• Vomiting and diarrhea (less common)

Sore throat, sneezing, stuffy nose, and stomach problems are less common flu symptoms. What some people call “stomach flu” is not influenza.

How the flu differs from a cold and COVID-19

The common cold, flu, and COVID-19 are respiratory illnesses caused by different viruses that can all cause similar symptoms. A cold is often milder than the flu. The flu and COVID-19 have similar symptoms. A change in or loss of taste or smell is more common with COVID-19.

Recognizing fever in older adults

Average normal body temperatures in older adults are slightly lower than in younger adults, so fever temperatures may also be lower when an older adult is sick. According to the U.S. Centers for Disease Control and Prevention (CDC), the following temperatures in older adults may indicate an infection such as the flu:

• A single temperature reading higher than 100°F (37.8°C)
• Multiple temperature readings above 99°F (37.2°C)
• A rise in temperature greater than 2°F (1.1°C) above the person’s normal body temperature

Related Health Effects

Anyone can develop the flu, but some people are more likely to experience complications. Complications can be mild, such as a sinus or ear infection, or more serious, like pneumonia.

Most deaths attributed to influenza virus are caused by pneumonia, including both viral and bacterial pneumonia. Because the body is fighting off the flu, a person may also develop a secondary infection such as pneumonia.

Flu vaccination has been linked to lower rates of heart problems (cardiac events) among people with heart disease and fewer hospitalizations among people with chronic lung disease or diabetes. It is important not to smoke when sick with the flu, as it is a respiratory illness that can infect the lungs as well as the nasal passages — the same areas affected by smoking.

Diagnosis

Influenza can be difficult to diagnose because its most visible symptoms (fever, cough, and aches) are similar to many other common illnesses. Healthcare providers have several ways to determine if a person has the flu.

Rapid Tests

Rapid influenza diagnostic tests can detect influenza in under 30 minutes, using swabs or samples of secretions taken from a person’s nose or throat. However, these tests can yield false positive or false negative results and may not identify the specific strain of flu.

Laboratory Tests

More accurate testing methods require healthcare providers to send samples from the nose, throat, or other biological materials to a lab.

Causes and Risk Factors

How the Flu Spreads

The flu is contagious. It spreads from person to person. It mostly spreads through droplets in the air when people with flu cough, sneeze, or talk, and it can spread from up to six feet away. Although less common, the flu can also spread from surfaces — for example, if a person touches something the virus is on and then touches their nose, mouth, or eyes.

It is possible to spread the flu before feeling sick and while having symptoms. Typically, people with the flu can spread it a day before, and up to a week after, feeling sick. Young children and people with weakened immune systems may spread the flu for even longer.

Who Is at Higher Risk for Complications

You are more at risk for flu and its complications if you:

• Are age 65 or older
• Have certain medical conditions such as asthma, diabetes,
  chronic kidney disease, obesity, heart disease, or have had
  a stroke
• Live in a nursing home or other long-term care facility
• Are pregnant
• Are a child younger than five
• Are immunocompromised

Prevention

Stop The Spread

You can help stop the spread of flu by:

• Washing your hands
• Covering your mouth when you cough or sneeze
• Avoiding touching your eyes, nose, or mouth
• Staying home when you are sick
• Avoiding close contact with people who are sick
• Cleaning and disinfecting frequently touched surfaces at home, work, or school

Get an Annual Flu Vaccination

The most effective way to prevent the flu is to get a flu vaccine every year. The U.S. Centers for Disease Control and Prevention recommends a yearly flu vaccine for everyone six months and older. Vaccination makes it less likely that you will develop the flu and reduces your chances of being hospitalized or dying if you do get sick.

You need a flu vaccine every year because:

• Flu viruses change, and the flu vaccine is updated each year to target the viruses anticipated to spread that season.
• The protection from a flu vaccine lessens with time, especially in older adults.

When to Get Vaccinated

You can develop the flu at any time of year, but it is most common in the fall and winter, typically starting to spread in October and peaking between December and February. It takes at least two weeks for a flu vaccine to start working, so try to get vaccinated by the end of October. Getting vaccinated after October can still help protect you. Because older adults may lose protection from a flu vaccine more quickly, try to avoid getting vaccinated too early — as in before September.

Where to get vaccinated

You can get your flu vaccine at a doctor’s office or from your local health department. Many grocery stores, drug stores, and pharmacies also offer flu vaccines. Visit Vaccines.gov to find a location near you. Children getting their first vaccine need 2 doses.

Flu vaccines for older adults and immunocompromised
 

The CDC recommends that people age 65 and older and people who are immunocompromised receive a higher-dose flu vaccine or an adjuvanted flu vaccine (one with an additional ingredient called an adjuvant that helps create a stronger immune response). These vaccines are potentially more effective than the standard flu vaccine for people in this age group.

Vaccine safety

The flu vaccine is safe and cannot give you the flu. Most people have no problems after getting a flu vaccine. When side effects occur, they are generally mild and go away on their own. The most common side effects are soreness, redness, or swelling at the injection site. Some people also develop a headache, fever, nausea, or muscle aches, which start shortly after vaccination and can last up to two days.

Even people with egg allergies can safely get most flu vaccines, and egg-free flu vaccines are also available. You should not get vaccinated if you have had a severe allergic reaction to the flu vaccine in the past.

Cost and access

Most people can get a flu vaccine for little to no out-of-pocket cost. Medicare and most private health insurance plans cover the cost of flu vaccination. If you do not have health insurance, contact your local or state health department.

Why the seasonal vaccine must be updated annually

As influenza circulates around the world each year, the virus’s genes mutate, causing proteins on the surface of the virus to structurally change. Over time, these small changes accumulate to cause “antigenic drift,” which enables the flu virus to partly evade protections built up through previous vaccinations or exposure. Scientists monitor changes in globally circulating flu strains and use this information to predict which strains are likely to be most prevalent during the next flu season. The seasonal flu vaccine is then designed to protect against three or four predicted dominant strains.

Treatment and Management

If you develop the flu, act fast and talk with a healthcare provider. Because the flu and COVID-19 have similar symptoms, you may need to get tested for an accurate diagnosis.

Antiviral Medicines

There are four antiviral drugs currently recommended for treating the flu: oseltamivir, zanamivir, peramivir, and baloxavir marboxil. These drugs work by interrupting the function of a protein on the virus surface (known as neuraminidase) and preventing the release of viral particles from infected host cells. They can treat both influenza A and B and have the biggest impact when taken within 48 hours of the onset of flu symptoms.

When taken promptly, antiviral drugs can reduce the severity of the flu’s worst symptoms and can shorten the length of the illness by an average of one day. Taking antiviral drugs early is especially important for people at high risk for flu complications, such as older adults or people with compromised immune systems.

Antivirals can also help reduce your risk of complications from flu, including bacterial pneumonia. Prescription antiviral drugs may be used to treat the flu in people who are very ill or who are at high risk of flu complications, but they are not a substitute for getting vaccinated.

Two older drugs — rimantadine and amantadine — were used to treat flu infection in the past, but several strains of flu have developed resistance to them. These two medicines are no longer recommended in the United States for treating the flu.

Antibiotics

Antibiotics do not help you recover from the flu. However, they are sometimes prescribed to treat a secondary infection if it is caused by bacteria.

Self-care

If you are sick with the flu, the following steps can help you feel better:

• Rest and drink plenty of fluids like juice and water (not alcohol).
• Use over-the-counter medicines such as acetaminophen or ibuprofen to bring down fever and relieve aches and pains. Avoid aspirin in children and teenagers due to the risk of Reye's Syndrome.
• Do not smoke: The flu is a respiratory illness that can infect your lungs as well as your nasal passages, the same areas affected by smoking.
• Stay home to avoid spreading the flu to others.

When to Contact a Healthcare Provider

Monitor your symptoms and contact a healthcare provider right away if you:

• Have shortness of breath or difficulty breathing
• Feel weak, dizzy, or confused
• Develop ongoing pain or pressure in the chest or abdomen
• Have a fever or cough that goes away and then comes back, which may be a sign of a secondary infection
• Experience worsening of other chronic health problems, such as asthma or heart disease
• Develop any other symptoms that worry you

Find Clinical Trials

Clinical trials uncover better ways to prevent, diagnose, treat, and understand diseases and conditions. To ensure results apply to everyone, volunteers of all ages, sexes, and backgrounds, including both healthy individuals and those with specific medical conditions, are needed. Find NIH Clinical trials on influenza.

Community Support

• Centers for Disease Control and Prevention has information on influenza.
• American Lung Association has information on influenza.
• U.S. Food and Drug Administration has information on flu vaccines.
• Vaccines.gov has information on where to get vaccines.

Note: This resource list is for informational purposes only. It is not comprehensive, and an organization’s inclusion does not constitute an endorsement by NIH.  

Research Information

The National Institutes of Health (NIH) is the largest public funder of biomedical research in the world. NIH invests most of its budget in medical research seeking to enhance life and to reduce illness and disability. NIH-funded research has led to breakthroughs and new treatments helping people live longer, healthier lives, and building the research foundation that drives discovery.

NIH conducts and supports research to find new and improved ways to diagnose, treat, and prevent influenza infection.

The Biology of Influenza Viruses

Influenza is challenging for scientists to study because there are hundreds of strains classified into four main categories: A through D, though D is not known to infect people. Influenza A virus most commonly causes illness in humans and is the source of all major influenza pandemics in modern history.

The surface proteins that combine in different ways to create influenza A strains are called hemagglutinin (HA) and neuraminidase (NA). Hemagglutinin enables the flu virus to enter a human cell and initiate infection; neuraminidase allows newly formed flu viruses to exit the host cell and multiply throughout the body. There are 19 types of HA and 11 types of NA, leaving the possibility for dozens of different subtypes of influenza A viruses — such as H1N1, H3N2, H5N8, and H7N9 — and strains.

Universal Flu Vaccine

A key focus of NIH’s influenza research program is developing a universal flu vaccine — one that could protect against multiple flu subtypes and eliminate the need for an annual seasonal flu vaccine.

H5N1

NIH scientists and NIH-supported researchers have been working to closely monitor H5N1 avian influenza, understand spread among animals, and develop ways to prevent and treat H5N1 influenza. For example, NIH-supported scientists have conducted tests to determine at what temperatures the virus is inactivated in milk and performed genetic mutation studies to understand the potential risk for spread from person-to-person. They are also conducting and supporting research to find potential therapeutics and advance new vaccine candidates.

Antiviral Drug Development

Because the influenza virus can develop resistance to antiviral drugs, NIH is working to find new and better treatments.

Other Research

NIH-supported researchers are conducting studies relevant to the flu, including whether probiotics can enhance older adults’ response to flu vaccine, meditation and exercise for preventing acute respiratory infection, and the effects of echinacea on the immune system.

Find NIH-funded research projects using NIH RePORTER, a searchable database of current and past research projects supported by NIH and other federal agencies.