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Influenza, Northern Hemisphere

Frequently Asked Questions (FAQ) about 2020-21 influenza vaccine availability 

Visit the 2020-2021 Seasonal Influenza Resource Center

Flu disease picture  Influenza is a highly contagious viral infection. There are three main antigen types of influenza: types A, B, and C. Influenza type A causes moderate to severe illness and affects all age groups. Type A can infect both humans and animals. The notion that influenza type B generally causes milder disease than type A has been recently challenged and is more fatal in children. Type B mainly infects humans. Influenza type C only causes mild respiratory illness. Influenza activity peaks from December to March in the Northern Hemisphere.

Influenza is transmitted from person to person mainly when an infected person coughs or sneezes. Transmission can also occur from direct contact with respiratory secretions, such as when touching surfaces contaminated with influenza virus and then touching the eyes, nose, or mouth, or from indirect contact with droplets from coughs or sneezes. Transmission can occur from the day before symptoms begin until about 5 days after symptoms begin in adults, or up to 10 days or more after symptoms begin in children.

About 50% of infected persons will develop "classic" influenza disease. This is characterized by the abrupt onset of fever, muscle pain, sore throat, nonproductive cough, runny nose, and headache. These symptoms generally last from 2 to 3 days. However, some people may experience a loss of strength or energy for several weeks. In addition, some people may develop complications, such as pneumonia.

Source: Centers for Disease Control and Prevention

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Sorry flu, not this year

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U.S. Air Force Kathryn Klein, right, an aerospace medical service specialist with 182nd Medical Group, Illinois Air National Guard, administers an influenza vaccination during drill weekend at the 182nd Airlift Wing in Peoria, Ill., Dec. 8, 2019. According to the Centers for Disease Control and Prevention, the flu is a contagious respiratory illness caused by influenza viruses, and the best prevention is getting a flu vaccine each year. (U.S. Air National Guard photo by Senior Airman Paul R. Helmig II)

The Defense Logistics Agency Troop Support Medical supply chain teamed with Department of Defense partners to provide 3.4 million doses of the influenza vaccine to service members, dependents and retirees.

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Navy Hospital Corpsman Kenny Liu, from San Jose, assigned to USS Gerald R. Ford's medical department, prepares a needle with a flu vaccination in the ship's hangar bay. (U.S. Navy photo by Mass Communication Specialist Seaman Apprentice Angel Thuy Jaskuloski)

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