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What is influenza?

Influenza is a serious respiratory disease caused by the influenza virus. Influenza viruses are classified as types A, B, or C. Influenza A is further classified into various subtypes. Influenza viruses of the A and B type are responsible for classic influenza illness. The disease is completely different from the common cold or the stomach flu. Influenza is a specific illness, but many people say that they “have the flu” to indicate that they are under the weather or ill in a general sort of way. Influenza produces widespread, sporadic respiratory illness each year during fall and winter in the Northern hemisphere. It has also been known to occur in epidemic or pandemic forms. The symptoms of influenza include the sudden, abrupt onset of chills and high fever in the range of 102°F to 103°F. Severe generalized aches and pains, often most intense in the back and the legs, accompany the fever. Exhaustion is common, as is headache and loss of appetite. Pain felt “behind” the eyeballs is often reported. Respiratory symptoms are initially mild; a scratchy or sore throat can accompany a slight dry cough. The lung symptoms develop later and come to dominate the picture with persistent and productive cough as an unmistakable feature. Fever can last up to 5 days in uncomplicated cases. After other symptoms subside, weakness and fatigue may linger for as long as several weeks.

Unfortunately, not all persons who become ill with influenza have an uncomplicated course as described above. Severe, fulminant, or fatal pneumonia can complicate influenza. Influenza is responsible for more than 35,000 deaths and 110,000 hospitalizations each year in the U.S. alone. Many of the deaths and hospitalizations occur among persons with underlying health conditions that place them at increased risk for complications. Increasing age (age 50 years or older), as well as very young age (age 2 years or younger) also place individuals at increased risk of influenza complications. Pregnancy is a common risk for influenza complications. Asthma is also considered a risk. Influenza can now be diagnosed in a few minutes in a doctor’s office. If you have symptoms of influenza, your doctor may advise you to have a nasal wash performed to be tested for the influenza virus. If you test positive for influenza, you might then be a candidate for immediate treatment with the antiviral medicines oseltamivir (Tamiflu), which is FDA approved for treatment of influenza for those aged 1 year and older, or zanamivir (Relenza), which is approved for older children and adults. Zanamivir is administered by inhalation and may be less desirable in asthmatic individuals. Both medicines shorten the time period during which you can transmit influenza to others and help to reduce the severity and duration of your illness, provided they are taken very early in the course of influenza. Oseltamivir, for example, is effective only if started within 48 hours of the onset of influenza symptoms. Taking either medicine later in the course of an established influenza infection does not provide benefit. Oseltamivir is also approved for the prevention (prophylaxis) of influenza in exposed and at-risk adults, as well as adolescents aged 13 years and older. Neither oseltamivir nor zanamivir are considered a substitute for influenza vaccination.

What is influenza?

The single best way to prevent influenza is vaccination (Table Recommendations for Influenza Vaccination). Vaccination against influenza virus causes the body’s immune system to manufacture protective antibodies. The antibodies produced in response to successful vaccination will help fight off influenza when and if a person becomes exposed to the virus. Detailed studies of the structure of the influenza virus reveal that the virus wraps itself in a protective envelope or coat. Different strains of influenza virus carry (or “express”) different proteins on the surface of the viral envelope or coat. Influenza is considered a tricky and clever virus because of its ability to change the proteins on its coat. Even subtle changes increase influenza’s ability to invade the human body and to cause illness. Antibodies directed against one specific strain of influenza are actually specific against that particular strain’s envelope proteins. So, the body needs to produce specific yet different antibodies against different influenza virus envelope proteins in order to effectively protect against different strains of virus. Antibodies against one strain are unlikely to offer protection against a different strain with a different coat or envelope. Different strains of influenza circulate each “flu season” which is why you can “get the flu” for two or more years in succession. It is also the reason that each year, governmental health agencies advise vaccine manufacturers on what strains to include in that particular year’s influenza vaccine.

Table Recommendations for Influenza Vaccination

Indications for Yearly Influenza Vaccination
Vaccination is the best way to prevent illness from influenza.* Asthma is an indication for yearly influenza vaccination, beginning at 6 months of age.
Who should receive the influenza vaccine?
• All adults, beginning at age 50
• Anyone aged 6 months or older with asthma
• Healthy babies and children aged 6 months to 23 months
• Pregnant woman who will be in their second or third trimester of pregnancy during influenza season
• Persons aged 6 months and older with a chronic respiratory (lung) or cardiovascular (heart) condition
• Persons aged 6 months and older with diabetes, chronic blood, kidney or immune system disease
• Nursing home and chronic-care facility residents
• A child or teen receiving long-term aspirin therapy
• Persons likely to transmit influenza to others at high risk for the development of influenza complications. The category includes health care providers (doctors, nurses), as well as caregivers and household contacts of persons who fall into any of the above categories.
• Anyone over the age of 6 months who wishes to reduce the likelihood of becoming ill with influenza
• Vaccination may be considered on a case-by-case basis for persons living in dormitories, or other crowded conditions (to prevent outbreaks); for persons providing essential community services (such as firefighters and police); for persons at high risk traveling to the Southern hemisphere (April to September), to the tropics, or in organized tourist groups (at any time).
Note: The above indications for influenza vaccination apply when vaccine supplies are adequate. If a shortage develops, healthy individuals aged 2 to 65 should forgo vaccination.
*Aperson who has experienced a significant egg allergy, an allergic reaction to a prior influenza vaccine or to any influenza vaccine constituent, should not receive the influenza vaccine.

A different influenza vaccine is thus developed and produced each year. The influenza vaccine available each fall in the United States is directed against three (3) specific influenza strains that health authorities believe will cause most of the serious illness for that year’s fall and winter influenza season. The vaccine you receive in the fall of 2005, for example, can be expected to protect you from the common disease-causing strains of influenza circulating in the United States during the winter of 2005 and 2006.

Two different types of influenza vaccines are available for prevention of influenza in the United States: the inactivated influenza vaccine, and the live, attenuated influenza vaccine (LAIV) (Table Comparison of Available Influenza Vaccines). The first is an inactivated vaccine.

Table Comparison of Available Influenza Vaccines

Inactivated Influenza Vaccine Live, Attenuated Influenza Vaccine  
• The vaccine is a “shot,” given by injection into muscle. • The vaccine is sprayed into the nostrils.  
• The vaccine has been used for many years. • The vaccine was licensed in the U.S. in 2003 for use in healthy children andadults from 5 to 49 years of age.  
• The vaccine contains an inactivated(“killed”) form of influenza virus.  
• The vaccine contains a live, attenuated (“weakened”) form of influenza virus.  
• The vaccine is updated every year.  
• The vaccine is best taken in October or November each fall. Vaccination is required yearly. • The vaccine is not presently recommended for pregnant women, and for people with underlying medical diseases. It is not presently recommended for persons with asthma.  
• Protection begins about 14 days after vaccination.  
• Protection lasts about 1 year. • The vaccine is updated each year.  
• October or November is the best time for vaccination. It should be repeated yearly.  
• Side effects, if any, are usually mild: soreness at the injection site is the most common. Fever and aches canoccur. Call your doctor if you think you are experiencing a more serious side effect.  
• Side effects, if any, are usually mild and may include: runny nose, congestion, cough, mild fever, aches, fatigue, throat soreness, and in children, abdominal pain, vomiting or diarrhea.  

“Inactivated” means that the vaccine contains a “killed” strain or form of influenza. It is given by injection into a muscle. The inactivated vaccine is the familiar “flu shot” that has been in clinical use for many years. The LAIV, or live, attenuated influenza vaccine, contains a weakened strain of influenza. The LAIV is administered intranasally, by spraying into the nostrils. The LAIV was first licensed in the United States in 2003. It is manufactured by Wyeth and is named Flu-Mist. Like the inactivated influenza vaccine, LAIV stimulates the body’s production of protective antibodies directed against three strains of influenza currently in circulation. Both vaccines are administered each year, ideally in October or November. LAIV, as of this writing, is only approved for administration to healthy persons between ages 5 and 49 years. It is not approved for use in pregnancy, nor is it approved in the setting of asthma at the present time.

Because influenza can be a fatal and devastating illness, especially in certain groups of individuals, vaccination is recommended for those at high risk of developing medical complications from infection with influenza. “Medical complications” is a term that encompasses hospitalization, severe illness manifestations, respiratory failure, and death. All persons with asthma, including children as well as adults, are candidates for yearly influenza vaccination. Unvaccinated healthy persons can get sick with influenza, and in turn pass the infection along to others. Only in a few, very specific circumstances is vaccination absolutely contraindicated. A person who has experienced a significant egg allergy, or an allergic reaction to a prior influenza vaccine or to any constituent of the vaccine, should not receive the influenza vaccine. Anyone who has been diagnosed with a neurological condition called Guillain-Barre Syndrome should consult closely with a physician knowledgeable about the risks of vaccination. In general, influenza vaccination is very safe and effective. You cannot get “the flu” from influenza vaccination, nor can the vaccination cause an infection. It is possible, however, to become ill with influenza even if you received the vaccine, as the vaccine does not protect against all strains of influenza. I remind my patients that the goal of influenza vaccination is to protect them from becoming severely ill from influenza. I still consider vaccination a success if they come down with a milder form of illness that slows them for a few days, but does not lead to an exacerbation of their asthma or to hospitalization.

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