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Influenza, or the "flu",  is caused by the influenza virus.  There are two types of influenza virus, type A and type B, and type A is further divided into subtypes based on two surface antigens (proteins which induce the immune response).  Yearly changes in the surface antigens affect a person's immunity, and large changes in the antigen will cause a person to become susceptible to the virus again.  Influenza viruses circulate predominantly in the winter months and are easily spread from person to person through the air or by direct contact.  Between ten and forty percent of children will contract influenza during any given winter, with the highest rates among school-age children.

In April 2009 a new strain of influenza type A, called the "novel H1N1" strain, was recognized in Mexico and has since spread around the United States and the world. The CDC now refers to "seasonal" influenza, which is comprised of the usual circulating influenza A and B viruses, and "novel" influenza, consisting of the new H1N1. Children and young adults seem to be particularly susceptible to the novel influenza viruses because of the lack of immunity, but thus far the virus appears to be acting like any other influenza virus: bad cold and flu-like symptoms that generally resolve over several days.

The incubation period of influenza is 1-4 days, with an average of 2 days.  Symptoms of influenza include the sudden onset of fever, headache, muscle aches, sore throat, and dry cough.  Other respiratory tract signs including sore throat and nasal congestion develop over the next several days.  Most children have a self-limited illness that subsides after several days, but influenza can exacerbate underlying lung or heart disease and cause either viral or secondary bacterial pneumonia.  The chances of pneumonia or other complications (such as myositis--inflammation of the muscle) are significantly higher for children with underlying chronic disease, particularly any longstanding lung disease (including asthma), diabetes, kidney disease, and hemoglobinopathies (congenital problem with the oxygen-carrying protein in blood cells).  Infants are also susceptible to prolonged influenza disease, although other viruses--particularly respiratory syncytial virus--are by far more likely causes of serious illness in very young infants.  In uncomplicated influenza, a person is contagious for approximately 24 hours before the onset of symptoms and 5 days after the onset of symptoms.

The hospitalization rate for influenza is about 1 in 1000 for children between 0 and 4 years, with the rate being five times higher for high-risk children.  Influenza is rarely fatal in children; it causes about 36,000 deaths a year in the United States, but >90% of these deaths are in older adults.  The death rate for influenza in children is 3.8 in 100,000.

Influenza can be diagnosed by rapid testing, although the test is not 100% accurate.  On occasion we may send your child to a laboratory for a rapid influenza test to confirm the diagnosis.  The influenza virus can be cultured, although the time it takes to grow the virus limits its usefulness in an office setting.

The best way to prevent influenza is to vaccinate.  Each year a new vaccine is produced based on what the anticipated strains of influenza viruses will be. The American Academy of Pediatrics now recommends that all children over six months of age receive the flu vaccine. The vaccine needs to be given yearly because the previous year's vaccines may not adequately protect against the current year's circulating influenza viruses."High risk" children especially recommended to receive flu vaccine include:

  • All children between 6 and 59 months due to increased risk of hospitalization and complications
  • Asthma or other chronic pulmonary disease such as cystic fibrosis or bronchopulmonary dysplasia
  • Heart disease that significantly affects the heart's ability to pump blood
  • Children with immune system defects or on chronic medications that affect the immune system
  • Diabetes
  • Chronic kidney disease
  • Hemoglobinopathies (congenital problem with the oxygen-carrying protein in blood cells), including sickle cell anemia
  • Diseases requiring long-term aspiring therapy, such as Kawasaki Disease
  • Any siblings of children with the above conditions

Seasonal influenza vaccine is now available (August 2009). Novel H1N1 vaccine, which is being produced separately, is currently being developed.

Influenza vaccine takes several weeks to become effective and should ideally be completed by the end of November.  Children under the age of 9 years receiving influenza vaccine for the first time need to have two doses spaced one month apart.

Side effects of the influenza vaccine are rare.  The influenza vaccine contains inactivated influenza virus and cannot cause influenza.  Fever in the first 24 hours is not uncommon under 24 months of age, and local reactions (redness, soreness, and warmth) occur in approximately 10% of adolescents.  The influenza vaccine is not recommended if your child has had a severe allergic reaction to egg (anaphylaxis); in the case of a minor egg allergy (hives), we may consider giving the influenza vaccine only if your child is high-risk and is thus likely to benefit from a flu shot.  In that case we ask that we observe your child for 30 minutes after the flu shot.  Influenza vaccine does not exacerbate asthma.

Some influenza vaccine contains thimerosal, a preservative containing mercury, but the dose is minute (12.5 micrograms) and well below the FDA guidelines of toxicity so it poses no risk to your child.

An intranasal vaccine called FluMist was licensed in 2003.  The vaccine is given by nasal spray, and is approved for healthy children two years and older.  The vaccine is a live virus vaccine, and as such is extremely effective in addition to being "child-friendly".  The vaccine is an excellent option for healthy children, particularly since universal flu vaccination is now recommended. The side effects of the vaccine are usually limited to a couple of days of congestion and a runny nose.

There are two antiviral medications approved for treatment of influenza: zanamivir (Relenza), and oseltamavir (Tamiflu).  Their usefulness is limited and resistance to Tamiflu has risen in the last year.  When given within 48 hours of the onset of symptoms, the medications do reduce the duration of influenza symptoms, but only to a small extent; zanamivir and oseltamavir will shorten the symptoms only by 1 day.  None of the antiviral medications will prevent the development of complications such as pneumonia.  Oseltamivir is approved down to 1 year of age, while zanamivir is only approved to 7 years of age and cannot be given to a child with underlying asthma.

For more information see the CDC influenza web page and the flu links at the American Academy of Pediatrics web page.

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