Influenza (Flu)

On this page

• Flu Symptoms
• Vaccines
• Testing
• Treatment

Influenza, or flu, is caused by influenza viruses. It is sometimes referred to as "seasonal influenza" because the virus can mutate, or change, and emerge in a slightly different form each year. These changes mean they are capable of causing illness even among those who have protective immunity from a vaccine or influenza illness in the past.

It is important to understand that Influenza (flu) is a potentially serious disease that can lead to hospitalization and sometimes even death.

Influenza viruses A and B circulate throughout the year, but in the United States, they cause the greatest number of illnesses during the fall and winter months which are known as "flu season." Influenza A viruses cause the most serious respiratory illness and, in addition to humans, are known to infect a wide range of mammals and aquatic birds (ducks, swans, etc.). Influenza B viruses infect only humans.

Influenza A viruses are named according to the variation of two proteins on their surface, hemagglutinin (H) and neuraminidase (N), each of which are numbered according to their structure, whereas Influenza B viruses are named according to their origin. Two types of Influenza A viruses (H1N1 and H3N2) and two types of Influenza B viruses (B/Victoria and B/Yamaguchi) have been responsible for all seasonal flu outbreaks in humans since 1977.

In addition, all influenza virus types mutate, or change, constantly, with new subtypes emerging every year. The fact that influenza viruses constantly mutate makes it especially important for employers to seek up-to-date information from public health authorities about the predominant circulating strains. This will enable employers to determine which workplace controls are best suited to address influenza-related hazards and share relevant information with employees, such as when new vaccine formulations become available. The CDC provides weekly updates about U.S. flu activity on its FluView webpage.

Influenza A viruses affecting animals, primarily birds and pigs, have mutated and infected humans. These "spillover" events have occurred several times in history, including the 1918 pandemic in which H1N1 was introduced into the human population. More recently, in 2009, a swine flu strain of H1N1 entered the human population causing severe illness and death. Currently, public health officials are monitoring H5N1, a highly pathogenic influenza A virus that is known to infect birds, and has infected dairy cattle and other mammalian species.

Flu Symptoms

Unlike the common cold, symptoms of flu usually appear rapidly, about 2 days after infection. These may include but are not limited to:

• Fever and/or chills*
• Cough
• Sore throat
• Runny/stuffy nose
• Body aches
• Headache
• Fatigue
• Diarrhea

*Fever may or may not be present

Flu may last for a few days or as long as two weeks. People start shedding influenza virus, which can then infect others, about 24 hours before they even get symptoms. This means it is hard to know when someone is infectious, so the protection offered by vaccination is very important. Most people who get flu will recover but some people have complications that can cause infections of the upper (sinuses, throat, middle ears) and lower (bronchioles, lungs) respiratory systems. In some cases, the virus causes life-threatening complications or death.

Complications of Infection with Influenza

In addition to the complications described for all respiratory diseases, adults with flu who also suffer from chronic and acute health conditions may become gravely ill. Possible serious complications can occur when the virus triggers inflammation which can cause illness or make existing illnesses much worse. These can impact multiple organ systems including:

• the respiratory system (asthma attacks, acute respiratory distress, or respiratory failure),
• the heart (myocarditis),
• the brain (encephalitis),
• and muscle tissues (myositis, rhabdomyolysis).

In addition, respiratory infections caused by influenza and secondary bacterial infections may lead to a cascade of negative events caused by infection of the bloodstream, called severe sepsis, and potentially septic shock, which is an extreme, life threatening inflammatory response to infection.

Vaccines

The CDC has stated that vaccination is the best way to prevent serious illness from seasonal flu. The CDC and the Advisory Committee on Immunization Practices (ACIP) recommend that adults and children over the age of six months get vaccinated against influenza annually. Staying up to date with annual vaccinations may prevent illness and reduce the risk of hospitalization if you do get the flu.

Each year, the influenza vaccine formulations are updated based on surveillance, laboratory results, and clinical study data collected by national influenza centers in 114 countries that are part of the WHO Global Influenza Surveillance and Response System (GISRS). The WHO consults with world experts to review the collected data and make recommendations about the composition of the flu vaccine for the upcoming year. In the U.S., the Food and Drug Administration (FDA) selects the influenza viruses that will comprise the annual vaccine for the upcoming year.

For more information about influenza vaccines, see CDC's Key Facts about Seasonal Flu Vaccine.

Testing

Influenza testing is widely available in the United States. The most accurate tests for influenza are performed using respiratory swabs taken at the point of care (doctor's offices, pharmacies, urgent care centers, etc.) and are generally performed in hospitals and commercial laboratories. The test most commonly used is a nucleic acid amplification test (NAAT) that detects influenza virus genetic material in upper or lower respiratory fluid.

In 2023, the FDA issued an emergency use authorization for the first over-the-counter test to detect Influenza A and Influenza B viruses. These tests are often combined with at-home COVID-19 tests and are now sold at pharmacies and other retailers. As recommended for all at-home diagnostic test kits, carefully follow manufacturer's instructions for handling, testing, and interpretation of results.

Treatment

Many workers will have flu symptoms that can be treated with over-the-counter medications. Workers aged 65 years and older are more likely to be hospitalized with flu, since many have underlying health conditions. Likewise, other adults with specific health conditions described in "Complications of Infection with Influenza" [in document link] are at higher risk of severe illness and may be prescribed antiviral drugs to prevent severe flu symptoms. These include:

• oseltamivir phosphate (Tamiflu® or its generic version); given in a pill or liquid
• zanamivir (trade name Relenza®); an inhaler for people with chronic lung disease
• peramivir (trade name Rapivab®); given intravenously for some hospitalized patients

Note that antiviral drugs that target influenza should be taken early in the infection and are most effective when taken within two days of becoming ill. Patients who are over 65 and/or have underlying medical conditions are prioritized for influenza antiviral treatment.

For more information about antiviral drugs used to treat influenza, see CDC's Treating Flu with Antiviral Drugs.