Influenza Prevention – Information

Influenza Prevention – influenza, commonly called the flu, is an upper respiratory infection that causes epidemics (widespread outbreaks of disease) every year.

In the United States each year about 20 percent of the population becomes ill with influenza (about 60 million people), and 30,000 to 40,000 people die as a result. Occasionally influenza occurs in a pandemic, in which people worldwide become ill. The most significant influenza pandemic in modern times was the Spanish influenza pandemic of 1918, which sickened 40 percent of the world population and caused more than 20 million deaths.

Other pandemics occurred in 1957 (Asian influenza) and 1968 (Hong Kong influenza). Outbreaks of Avian flu created concern among public health officials in the early 2000s but containment efforts prevailed and limited the numbers of people who became ill.

Influenza Virus Strains: Moving Targets

Viruses cause influenza. There are three types of influenza viruses: influenza A, influenza B, and influenza C. Influenza A and B are responsible for most cases of illness; influenza C infections are generally mild and not so easily spread from one person to another as are A and B influenza viruses. Every year the strains of the influenza virus responsible for causing illness are slightly different from the strains that caused infection the previous year (epidemiologists call this “drift”).

These changes help the virus survive. Having an influenza infection confers immunity against the strain of virus that caused it. Because the strains vary each year, however, this immunity has value only for the duration of the flu season in which the virus strain is active (though a small amount of resistance may carry over to similar strains). Epidemiologists and researchers attempt to predict which strains will emerge each year, and base annual influenza vaccines on those strains.

Occasionally the influenza virus makes a dramatic alteration, a phenomenon epidemiologists call “shift.” These are the influenza viruses capable of causing pandemic, or worldwide, infection because no immunity exists against them. Health organizations around the world have monitoring systems in place to detect these viruses and respond before pandemic infection develops.

Because bird populations serve as reservoirs for influenza viruses that can also infect humans, health officials closely monitor avian influenza infections among birds. Avian influenza outbreaks among domesticated birds in parts of Asia in the late 1990s and early 2000s caused alarm for the potential of a pandemic, though containment responses were effective in confining the outbreaks. The ease with which people travel around the world creates considerable challenge for containing outbreaks.

Influenza Vaccination

Vaccines provide immunity by stimulating the immune system enough to produce antibodies to fight the virus at its next attempt to enter the body but not enough to cause illness. The resulting immunity is effective against only the specific strain of virus. Two kinds of influenza vaccines are available in the United States:

  • The conventional flu shot contains inactivated (killed) influenza virus, which, when injected into the body, cause the immune system to respond. The first killed-virus influenza vaccine became available in the United States in 1945. Anyone older than six months of age can receive the flu shot.
  • The live attenuated vaccine, which comes in the form of a nasal spray, contains live but weakened influenza virus genetically altered so it cannot cause illness (it is unable to survive at body temperature). The weakened virus enters the bloodstream via the mucous membranes of the nasal passages. Like the inactivated influenza injected vaccine, the live attenuated vaccine activates an immune system response to produce antibodies. The live attenuated influenza vaccine became available in the United States in 2003. Only people between the ages of 5 and 49 who are healthy can receive the live attenuated virus.

Health experts recommend getting the influenza vaccine in October or November, as the flu season in the United States typically runs December to March each year. Though everyone can benefit from vaccination, certain groups of people are at high risk for infection. They include

  • children between the ages of six months and 2 years as well as their household members
  • people age 50 and older
  • people who live in extended-care facilities and other group settings
  • people who work in health-care and public safety positions
  • people over age six months who have chronic health conditions

Occasionally there are shortages of vaccine, as occurred in 2004, which is a significant public health issue. When this occurs, public health agencies such as the US Centers for Disease Control and Prevention (CDC) and the US Department of Health and Human Services (HHS) issue revised guidelines to protect those who are most vulnerable to complications.

Antiviral Medications

Because influenza is a viral infection, most treatment measures are supportive and target symptoms. From a prevention standpoint, antiviral medications that can reduce the severity of symptoms can also reduce the spread of influenza infection. A doctor must prescribe an antiviral medication within 48 hours of the onset of symptoms; the more quickly after exposure, the more effective the medication. Antiviral medications available in the United States include amantadine (Symmetrel), rimantadine (Flumadine), zanamivir (Relenza), and oseltamivir (Tamiflu).

Frequent hand washing and sneezing or coughing into a tissue or the sleeve rather than the hands are among the most effective measures for preventing the spread of the influenza virus from person to person.


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