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Influenza: Influenza, commonly called "the flu," is an illness caused by viruses that infect the respiratory tract. Compared with most other viral respiratory infections, such as the common cold, influenza infection often causes a more severe illness.

Causes of influenza:

The Influenza VirusesInfluenza viruses are divided into three types, designated A, B, and C. Influenza types A and B are responsible for epidemics of respiratory illness that occur almost every winter and are often associated with increased rates of hospitalization and death. Influenza type C differs from types A and B in some important ways. Type C infection usually causes either a very mild respiratory illness or no symptoms at all; it does not cause epidemics and does not have the severe public health impact that influenza types A and B do. Efforts to control the impact of influenza are aimed at types A and B, and the remainder of this discussion will be devoted only to these two types. Influenza viruses continually change over time, usually by mutation. This constant changing enables the virus to evade the immune system of its host, so that people are susceptible to influenza virus infection throughout life. This process works as follows: a person infected with influenza virus develops antibody against that virus; as the virus changes, the "older" antibody no longer recognizes the "newer" virus, and reinfection can occur. The older antibody can, however, provide partial protection against reinfection. Currently, three different influenza strains circulate worldwide: two type A viruses and one type B. Type A viruses are divided into subtypes based on differences in two viral proteins called the hemagglutinin (H) and the neuraminidase (N). The current subtypes of influenza A are designated A(H2N1) and A(H3N2). Influenza type A viruses undergo two kinds of changes. One is a series of mutations that occur over time and cause a gradual evolution of the virus. This is called antigenic "drift." The other kind of change is an abrupt change in the hemagglutinin and/or the neuraminidase proteins. This is called antigenic "shift." In this case, a new subtype of the virus suddenly emerges. Type A viruses undergo both kinds of changes; influenza type B viruses change only by the more gradual process of antigenic drift.

Symptoms of Influenza:

Typical clinical features of influenza include fever (usually 100° F to 103° F in adults and often even higher in children) and respiratory symptoms, such as cough, sore throat, runny or stuffy nose, as well as headache, muscle aches, and often extreme fatigue. Although nausea, vomiting, and diarrhea can sometimes accompany influenza infection, especially in children, gastrointestinal symptoms are rarely prominent. The term "stomach flu" is a misnomer that is sometimes used to describe gastrointestinal illnesses caused by other microorganisms. Most people who get the flu recover completely in 1 to 2 weeks, but some people develop serious and potentially life-threatening medical complications, such as pneumonia. In an average year, influenza is associated with about 20,000 deaths nationwide and many more hospitalizations. Flu-related complications can occur at any age; however, the elderly and people with chronic health problems are much more likely to develop serious complications after influenza infection than are younger, healthier people.


Influenza Vaccine

Much of the illness and death caused by influenza can be prevented by annual influenza vaccination. Influenza vaccine is specifically recommended for those who are at high risk for developing serious complications as a result of influenza infection. These high-risk groups include all people aged 65 years or older and people of any age with chronic diseases of the heart, lung or kidneys, diabetes, immunosuppression, or severe forms of anemia. Other groups for whom vaccine is specifically recommended are residents of nursing homes and other chronic-care facilities housing patients of any age with chronic medical conditions, and children and teenagers who are receiving long-term aspirin therapy and who may therefore be at risk for developing Reye syndrome after an influenza virus infection. Influenza vaccine is also recommended for people who are in close or frequent contact with anyone in the high-risk groups defined above. These people include health care personnel and volunteers who work with high-risk patients and people who live in a household with a high-risk person. Although annual influenza vaccination has long been recommended for people in the high-risk groups, many still do not receive the vaccine. Some people are not vaccinated because of misperceptions about influenza and the vaccine. They mistakenly perceive influenza as merely a nuisance and believe that the vaccine causes unpleasant side effects or that it may even cause the flu. The truth is that influenza vaccine causes no side effects in most people. The most serious side effect that can occur after influenza vaccination is an allergic reaction in people who have severe allergy to eggs, since the viruses used in the vaccine are grown in hens' eggs. For this reason,
people who have an allergy to eggs should not receive influenza vaccine. Also, the vaccine is not recommended while individuals have active infections or active diseases of the nervous system. Less than one-third of those who receive vaccine have some soreness at the vaccination site, and about 5% to 10% experience mild side effects, such as headache or low-grade fever for about a day after vaccination. These side effects are most likely to occur in children who have not been exposed to influenza virus in the past. Nevertheless, some older people remember earlier influenza vaccines that did, in fact, produce more unpleasant side effects. Vaccines produced from the 1940s to the mid-1960s were not as highly purified as modern influenza vaccines, and it was these impurities that caused most of the side effects. Since the side effects associated with these early vaccines, such as fever, headache, muscle aches and fatigue, were similar to some of the symptoms of influenza, people believed that the vaccine had caused them to get the flu. However, influenza vaccine produced in the United States has never been capable of causing influenza. The only type of influenza vaccine that has been licensed in the United States up to the present time is made from killed influenza viruses, which cannot cause infection. An influenza vaccine that is made with live influenza viruses has been developed and may be marketed in the future. This vaccine is made with viruses that can confer immunity but do not cause classic influenza symptoms. Some people do not receive influenza vaccine because they believe it is not very effective. There are several different reasons for this belief. People who have received influenza vaccine may subsequently have an illness that is mistaken for influenza, and they believe that the vaccine failed to protect them. In other cases, people who have received vaccine may indeed have an influenza infection. Overall vaccine effectiveness varies from year to year, depending upon the degree of similarity between the influenza virus strains included in the vaccine and the strain or strains that circulate during the influenza season. Because the vaccine strains must be chosen 9 to 10 months before the influenza season, and because influenza viruses mutate over time, sometimes mutations occur in the circulating strains between the time vaccine strains are chosen and the next influenza season is over. These mutations sometimes reduce the ability of the vaccine-induced antibody to inhibit the newly mutated virus, thereby reducing vaccine efficacy.  Vaccine efficacy also varies from one person to another. Studies of healthy young adults have shown influenza vaccine to be 70% to 90% effective in preventing illness. In the elderly and those with certain chronic medical conditions, the vaccine is often less effective in preventing illness than in reducing the severity of illness and the risk of serious complications and death. Studies have shown the vaccine to reduce hospitalization by about 70% and death by about 85% among the elderly who are not in nursing homes. Among nursing home residents, vaccine can reduce the risk of hospitalization by about 50%, the risk of pneumonia by about 60%, and the risk of death by 75% to 80%. When antigenic drift results in the circulating virus becoming different from the vaccine strain, overall efficacy may be reduced, especially in preventing illness, but the vaccine is still likely to lessen the severity of the illness and to prevent complications and death.

Why Must The Vaccine Be Taken Every Year?

Although only a few different influenza viruses circulate at any given time, people continue to become ill with the flu throughout their lives. The reason for this continuing susceptibility is that influenza viruses are continually changing, usually as a result of mutations in the viral genes. Currently, there are three different influenza virus strains, and the vaccine contains viruses representing each strain. Each year the vaccine is updated to include the most current influenza virus strains. The fact that influenza viruses continually change is one of the reasons vaccine must be taken every year. Another reason is that antibody produced by the host in response to the vaccine declines over time, and antibody levels are often low one year after vaccination.

When should you receive the influenza vaccine?

In the United States, influenza usually occurs from about November until April. Typically, activity is very low until December, and peak activity most often occurs between January and March. Influenza vaccine should be administered between September and mid-November. The optimal time for organized vaccination programs for persons at high risk for influenza-related medical complications is usually the period from October to mid-November. It takes about 1 to 2 weeks after vaccination for antibody against influenza to develop and provide protection. Who is at increased risk for complications of influenza by virtue of age or underlying medical condition. Groups at increased risk of influenza complications include:       1. Persons aged 65 years or older.     2. Residents of nursing homes and other chronic care facilities housing patients of any age who have chronic medical conditions.     3. Adults and children with chronic disorders of the pulmonary or cardiovascular systems, including children with asthma.     4. Adults and children who have required regular medical follow-up or hospitalization during the preceding year because of chronic metabolic diseases (including diabetes mellitus), renal dysfunction, hemoglobinopathies, or immunosuppression.     5. Children and teenagers (6 months to 18 years) who are receiving long-term aspirin therapy and therefore may be at risk for developing Reye syndrome after influenza.     6. There is some evidence to suggest that women who are in the third trimester of pregnancy or in the early  postpartum period may be at increased risk for serious medical complications after influenza infection. Pregnant women who will be in the third trimester of pregnancy between December and April should consult their health care provider about receiving influenza vaccine during the period September to mid-November. In addition, the following groups should be vaccinated because, while not at high risk themselves, they may transmit influenza to persons who are at high risk for complications:     1. Physicians, nurses, and other health care personnel in both hospital and outpatient-care settings.     2. Employees in nursing homes and chronic-care facilities who have contact with patients or residents.     3. Providers of home care to persons at high risk--for example, visiting nurses and volunteer workers.     4. Household members (including children) of high-risk persons. Finally, influenza vaccine may be administered to any person who wishes to reduce his or her chances of acquiring influenza infection. Persons who provide essential community services may be considered for vaccination to minimize disruption of essential activities during influenza outbreaks. Students or other persons in institutional settings, such as those who reside in dormitories, should be encouraged to receive vaccine to minimize the disruption of routine activities during epidemics. Persons needing further information regarding the use and availability of influenza vaccine should consult with their health provider or their local health department.
Antiviral Agents:Although vaccination is the primary method for control of influenza, antiviral agents have a role in the prevention and treatment of influenza type A infection. Amantadine and Rimantadine are chemically related drugs that interfere with the replication cycle of influenza type A viruses. They are not effective against influenza type B or other viral respiratory pathogens. Safety of Amantadine and Rimantadine: Amantadine and Rimantadine can cause adverse reactions in some persons; such adverse reactions are rarely severe, but may be important for some categories of patients. Serious side effects, such as marked behavioral changes, delirium, hallucinations, agitation, and seizures, have been observed. These more severe side effects have been associated with high plasma drug concentrations and have been observed most often in elderly persons, and persons with renal insufficiency, seizure disorders, certain psychiatric disorders who have been taking amantadine at a dose of 200 mg per day. Clinical observations and studies have provided evidence that lowering the dosage of amantadine in these persons will reduce the incidence and severity of such side effects; recommendations for reduced dosages for these groups of patients have been made. Modifications in dosage of amantadine or Rimantadine may be required for persons with impaired renal or hepatic function, the elderly, children, and persons with a history of seizures. The package insert should be reviewed before use of amantadine or Rimantadine for any patient.

Clinical Effectiveness:

  • Prevention: Both amantadine and Rimantadine are approximately 70% to 90% effective in preventing illnesses caused by naturally occurring strains of type A influenza viruses.
  • Treatment: In otherwise healthy adults, amantadine and Rimantadine have been shown to reduce the severity and duration of signs and symptoms of influenza A illness when administered within 48 hours of illness onset.
Antiviral Resistance: Amantadine- and Rimantadine-resistant influenza A viruses can emerge when either of these drugs are administered for treatment; amantadine-resistant strains are cross-resistant to Rimantadine. There is no evidence that amantadine- or Rimantadine-resistant viruses are more virulent or more transmissible than amantadine-or Rimantadine-sensitive viruses. Resistant viruses have most frequently been isolated from persons taking one of these drugs as therapy for influenza A infection. Resistant viruses have sometimes been isolated from people who live in residential or institutional settings where other residents are taking or have recently taken amantadine or rimantadine as therapy. Persons with influenza-like illness should, to the extent possible, avoid contact with uninfected persons, whether or not they are taking amantadine or Rimantadine as treatment. It is advisable to discontinue amantadine or Rimantadine treatment of persons who have influenza-like illness as soon as clinically warranted, generally after 3-5 days of treatment or within 24-48 hours after the disappearance of signs and symptoms.

Influenza At A Glance:
  • Influenza, commonly called 'the flu', is caused by viruses that infect the respiratory tract.
  • Influenza viruses are divided into three types, designated A, B, and C.
  • Most people who get the flu recover completely in 1 to 2 weeks, but some people develop serious and potentially life- threatening medical complications, such as pneumonia.
  • Much of the illness and death caused by influenza can be prevented by annual influenza vaccination.
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