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Influenza is a highly infectious viral illness. It can cause mild to severe illness, and at times can lead to death.1

Most experts think that influenza viruses are spread mainly by droplets made when people with flu cough, sneeze or talk. These droplets can land in people’s mouths or noses, or can be inhaled into the lungs. Less often, a person might get influenza by touching a surface or object that has the virus on it and then touching his or her own mouth or nose.2

Most healthy adults may be able to infect other people with flu beginning 1 day before symptoms develop and up to 5 to 7 days after becoming sick. Children may pass the virus for longer than 7 days.2

Symptoms start 1 to 4 days after the virus enters the body, though some people can be infected with the flu virus and show no symptoms. Despite not showing symptoms, people may still spread the flu virus to others.2

While there are several types of influenza viruses, two are predominantly associated with human infection: type A and type B.3, 5 These viruses can severely impact more vulnerable populations, such as adults age 65 and older, as well as young children and babies.4

Type A viruses specifically have different subtypes based on the two different proteins of which they express on their surface: hemagglutinin (H) and neuraminidase (N).5 Hence, Influenza A (H3N2) is a type A virus that has H3 and N2 on its surface. Over time as the virus replicates, small changes in the genes of influenza can accumulate, resulting in viruses that are antigenically different.6 As a result of this antigenetic drift we have to change our flu vaccines every season in accordance with the drifting virus.6 Of note, occasionally more abrupt changes in the influenza virus can occur as well, known as antigenic shift, resulting in new hemagglutinin and/or new hemagglutinin and neuraminidase proteins.6

Disease Presentation in Older Adults

The clinical presentation of influenza ranges from asymptomatic infection or a self-limiting upper respiratory tract infection, to a severe illness with potentially fatal complications.1

In older adults, influenza sometimes presents differently than it does in other age groups:

Older adults may experience malaise, instead of the sudden onset of high fever typical in children and younger adults.7

Stomach pain, diarrhea, and nausea are more frequent symptoms in older adults than in other age groups.7

Runny nose, sore throat, and nasal congestion are all less frequent symptoms in older adults than in other age groups.7


Complications from influenza can lead to life-threatening conditions in older adults. Serious complications include1:

Myocarditis, encephalitis, myositis, or rhabdomyolysis
Multi-organ failure (e.g., respiratory and kidney failure)
Respiratory tract infection leading to an extreme inflammatory response and sepsis

1. Flu Symptoms & Complications. Centers for Disease Control and Prevention. 2. How Flu Spreads. Centers for Disease Control and Prevention. 3. Influenza (Flu) Viruses. Centers for Disease Control and Prevention. 4. People at High Risk of Developing Flu–Related Complications. Centers for Disease Control and Prevention. 5. CDC Yellow Book. Infectious Diseases Related to Travel. Influenza. Centers for Disease Control and Prevention. 6. How the Flu Virus Can Change: “Drift” and “Shift”. Centers for Disease Control and Prevention. 7. Call to Action: Reinvigorating Influenza Prevention in U.S. Adults Age 65 Years and Older. National Foundation for Infectious Diseases. 8. Flu View Weekly U.S. Influenza Surveillance Report. Centers for Disease Control and Prevention. 9. The Flu Season. Centers for Disease Control and Prevention.

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