What happens in an influenza outbreak?
Influenza is an acute (abrupt) respiratory disease. It is highly contagious and is now very much a disease of global importance.
The word "influenza" is menacing to those who know of the great pandemics of this century. These pandemics have occurred for centuries, three times -- 1918, 1957 and 1968 -- in the 20th century alone. The infamous "Spanish flu" of 1918 was responsible for more than 20 million deaths, primarily among young adults.
A pandemic is an epidemic, a sudden outbreak, that becomes extremely widespread and affects a whole region, an entire continent, or the world.
Influenza viruses are unique
Influenza is an extraordinary infectious disease. It emerges and reemerges. Influenza causes an annual epidemic that is due to influenza viruses that reemerge, having mutated (changed) enough so they can reinfect people who had an earlier bout of flu. And pandemics of influenza can strike unpredictably because of the emergence of new types of influenza virus that are harbored in birds and other animals.
Influenza viruses are unique in their ability to cause sudden, pervasive illnesses in all age groups on a global scale. With the frequency of global travel the threat of a pandemic is all the more real and frightening.
Flu Season Starts in the Fall
Influenza is not a minor illness. It is a serious disease that is caused by a virus that spreads from infected persons to the nose or throats of others. Influenza can cause: fever, cough, chills, sore throat, headache and muscle aches.
Most people are ill with influenza for only a few days, but some get much sicker and may need hospitalization. Influenza causes thousands of deaths each year, mostly in the elderly.
The influenza season (otherwise known as the flu season) in the U.S. usually starts in November and runs to March or April each year.
How Does Influenza Get Global?
An outbreak of influenza can start in a remote part of China and quickly spread throughout Asia. Sick travelers from Asia may fly to Los Angeles, Chicago, Honolulu and New York potentially infecting anyone they come in contact with. Soon, new outbreaks of influenza will be identified in whatever city the travelers visit. In a short time the virus could be spread throughout the U.S and to Europe as more sick travelers fly throughout the world. It is this way that influenza reaches global and pandemic proportions. Typically, influenza spreads from the southern hemisphere because the appearance of influenza viral strains there precedes their appearance in the U.S. The U.S. bases its annual influenza vaccine on the strains of influenza viruses circulating in the southern hemisphere during our spring and summer. This is flu season (fall and winter) in the southern hemisphere.
How is the Flu Vaccine Designed?
A World Health Organization (WHO) network for half a century has monitored the emergence of new types of influenza virus that may cause the next epidemic or pandemic of influenza. This network now includes about 110 laboratories in more than 80 countries around the world as well as 4 international reference labs.
The main aim of the WHO network is to match the recommended vaccine viruses as closely as possible with the influenza viruses that are the most likely to circulate during the next flu season. The vaccines recommended by the network in recent years have usually been well matched and therefore quite effective.
Who Should Have a Flu Shot?
The flu vaccine is generally recommended for persons in the following groups:
- Adults 65 years of age and older.
- Residents of nursing homes or other facilities for patients with chronic medical conditions.
- Persons over 6 months of age with:
- Chronic heart or circulation problems or
- Lung disorders, including asthma.
- Persons over 6 months of age with:
- Women who will be in their 2nd or 3rd trimester of pregnancy during the flu season.
- Persons 6 months to 18 years of age receiving long-term aspirin therapy.
- Groups, including household members and care givers, who can infect high risk persons.
- Persons traveling to foreign countries, depending on the season and destination, should consider vaccination (at least 2 weeks in advance).
- Anyone over 6 months of age who wishes to reduce the likelihood of becoming ill with influenza should be vaccinated.
When Should I Get the Influenza Vaccine?
The best time to get an influenza vaccine is between September and December in the northern hemisphere including the U.S. and Canada.
"I Had a Flu Shot Last Year. Do I Need Another?"
Yes. A new shot is needed each year. Protection develops about 2 weeks after the shot. It may last up to a year.
Medically reviewed by Robert Cox, MD; American Board of Internal Medicine with subspecialty in Infectious Disease
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