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Influenza

Backgrounder

Influenza, commonly referred to as "the flu," is a highly contagious viral disease and one of the most severe respiratory illnesses of the winter season. It causes an average of 36,000 deaths and more than 200,000 hospitalizations in the U.S. every year. Combined with pneumonia, influenza is the seventh leading cause of death in the nation. Influenza can lead to serious complications include bacterial pneumonia, dehydration, and worsening of chronic medical conditions, such as congestive heart failure, asthma or diabetes.

Influenza spreads easily from person to person, primarily when an infected individual coughs or sneezes. The virus can be transmitted even before influenza symptoms appear and for many days after the symptoms begin. Typical influenza symptoms include abrupt onset of high fever, muscle and joint pain, chills, a dry cough, headache, runny nose and a sore throat.

Children may have symptoms that are uncommon in adults, including diarrhea, vomiting or nausea. Hospitalizations and deaths are often related to bacterial infections that complicate the primary infection. Even among individuals who are not hospitalized, influenza can cause extreme fatigue that may last days or weeks.

Influenza viruses are divided into three types, influenza 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 C is rarely reported in humans.

Influenza Among Infants and Children

Rates of influenza infection are highest among children. Moreover, those younger than 24 months are hospitalized with influenza-related complications at rates similar to those of people 65 years and older.

Additional findings show that, for every 100 children younger than 15 years of age, 6 to 15 outpatient visits are attributable to influenza. Children with certain underlying medical conditions also face increased risks from influenza infection. These children are five times more likely than healthy children of the same age to be hospitalized with influenza-related illnesses. High-risk conditions among children include asthma and diabetes.

Influenza can also be fatal in children. Some seasons are marked by more severe influenza infections while others are relatively mild. Overall, it is estimated that, on average, 92 U.S. children younger than 5 years of age die each year from influenza-related complications. During the 2003-2004 season 152 deaths in children younger than 18 were reported.

Influenza Infection in Health Care Workers

The U.S. Centers for Disease Control and Prevention (CDC) has long recommended annual influenza vaccination for all health care workers, yet fewer than 4 in 10 health care workers are immunized annually. Health care workers infected with influenza can transmit the highly contagious virus to patients in their care, who may be at high risk for influenza-related complications and death.

Research suggests unvaccinated health care workers can be a key cause of outbreaks in a variety of health care settings. Institutional influenza outbreaks can have serious implications - patients are at risk of contracting influenza; staff shortages can result or be exacerbated; admissions may be curtailed; and costs may be increased.

In addition to recommending that all health care workers should be vaccinated against influenza annually, CDC strongly encourages facilities that employ health care to provide vaccine to workers by using approaches that maximize immunization rates.

Protection through Vaccination

Immunization provides the best protection against influenza. The optimal time to get vaccinated is October or November, but vaccination continues to be useful in December, January and even later because influenza can continue to circulate through March and beyond. The degree of vaccine effectiveness depends on several factors, including the age and health of the person being vaccinated and the match between circulating virus strains and those included in the vaccine.

Health authorities recommend influenza vaccination for certain people every year:

  • Children 6 through 23 months of age*
  • Persons 50 years of age and older
  • Persons aged 2 to 64 years with high-risk conditions, such as asthma, diabetes or heart disease
  • Children 6 months to 18 years of age who are on long-term aspirin treatment
  • Residents of long-term care facilities and nursing homes
  • Women who will be pregnant during influenza season
  • Health care workers who come in contact with patients
  • Household contacts and out of home caregivers of anyone in a high-risk group, including children less than 6 months of age who are too young to be vaccinated themselves
  • Anyone who wants to prevent influenza

Individuals with severe hypersensitivity to eggs or those who have had a previous vaccine-associated allergic reaction should avoid immunization.

Each year, a new influenza vaccine is formulated to protect against predominant circulating influenza strains. The strains included in the vaccine are those that scientists at the U.S. Food and Drug Administration (in cooperation with the World Health Organization) have determined will be the most likely predominant strains during the next influenza season.

Because circulating strains change constantly, it is not unusual that in some years the circulating influenza virus strains may not match exactly those contained in the vaccine. However, research has shown the vaccine is still protective against infection and reduces severity of influenza-associated complications.

Influenza vaccine prevents influenza infection in 70 to 90 percent of healthy adults less than 65 years of age. The vaccine is less effective at preventing infection in older persons (more than 65 years of age). However, among elderly persons not living in nursing homes or similar chronic-care facilities, influenza vaccine is 30 to 70 percent effective in preventing hospitalization for pneumonia and influenza. For the elderly living in nursing homes or other long-term care environments, the vaccine is 80 percent effective in preventing death from infection.

Despite the serious health threat, missed opportunities for influenza vaccination are common among children with chronic medical conditions. For example, more than eight million infants and children with asthma should receive influenza vaccine each year. However, nearly 70 percent do not receive the vaccine - this is the lowest vaccination rate for any recommended childhood vaccine in the U.S. Children can be immunized at the same time they are given other routine vaccinations during the influenza season.

There are two types of influenza vaccine available: the injectable trivalent inactivated influenza vaccine (TIV) and live attenuated influenza vaccine (LAIV) which is administered as a nasal spray.

TIV has been used safely and effectively for decades. To manufacture vaccine, influenza viruses are grown in chicken eggs, harvested and then chemically inactivated. The vaccine is purified and tested for purity, safety and its ability to stimulate protective antibody in humans. The influenza virus used in the vaccine is "killed" and cannot cause influenza. The vaccine is approved for use in anyone 6 months of age and older, regardless of health status.

LAIV, or nasal vaccine, was licensed for use in 2003. It is approved or use only in non-pregnant persons aged 5-49 years who do not have medical problems such as asthma, diabetes, heart disease or other chronic conditions.

Vaccine Safety

Individuals with severe hypersensitivity to eggs or those who have had a previous vaccine-associated allergic reaction should avoid immunization. Persons with acute high fever should usually wait until their symptoms subside. However, vaccination can proceed during minor illnesses, with or without fever, particularly among children with mild upper respiratory tract infections or hay fever.

The most frequent adverse effect of the injectable influenza vaccine is soreness at the injection site for one to two days. Occasionally, some people experience a period of mild fever and fatigue for a day or two after vaccination. The injectable vaccine is made from an inactivated, or dead, virus and cannot transmit infection.

Studies show that side effects from the nasal influenza vaccine are generally mild and temporary. The most common is runny nose; others include various cold-like symptoms, such as headache, cough, sore throat, tiredness/weakness, irritability and muscle aches. There is a potential risk of spread of vaccine-strain influenza virus to immunocompromised contacts.

Treating Influenza

Antiviral drugs are not a substitute for vaccination but in certain situations (i.e., outbreaks in institutions) they may be extremely helpful in preventing and controlling influenza.

Zanamivir and oseltamivir are antiviral drugs that have activity against both influenza A and B viruses. Both zanamivir and oseltamivir were approved in 1999 for treating uncomplicated influenza infections and so far, there has been no evidence of resistance of the influenza virus against these drugs. They need to be given within two days of illness onset to otherwise healthy adults to be effective.

Amantadine and rimantadine the two other antiviral drugs, are active against influenza A virus, but not influenza type B. When taken within 48 hours of the start of symptoms, these drugs can reduce the length of uncomplicated influenza A illness by approximately one day. One concern with these drugs, however, is that influenza viruses develop resistance to them. Because the influenza virus varies each season, some anti-viral drugs may not be recommended in a given season due to the virus strains' resistance to the drugs. Infection resistance should be discussed with a physician before taking an anti-viral drug for influenza. The CDC also provides annual guidelines.

Adjuctive treatment approaches for influenza include use of aspirin (for people 18 years of age and older), acetaminophen or ibuprofen; eating well and consuming liquids; and rest. Staying away from the workplace and other social situations during the early part of the illness may help to reduce spread of the virus to contacts. People with influenza are advised to see their physician if their symptoms worsen.