Protecting against influenza

Oct. 1, 2004
Influenza ("the flu") is a contagious respiratory illness caused by two types of influenza viruses. Symptoms range from mild to severe with life threatening complications.

Charles John Palenik, MS, PhD, MBA

Influenza ("the flu") is a contagious respiratory illness caused by two types of influenza viruses. Symptoms range from mild to severe with life threatening complications. In the United States, the peak of the flu season occurs during late December through March. The impact of a flu season varies each year.

On average, 114,000 people are hospitalized for flu-related complications each year. Over 36,000 people in the United States die annually due to influenza. Hospitalizations and deaths are most common among those over 65 years old, young children, and all ages with underlying health conditions, such as congestive heart failure, asthma, or diabetes.

Symptoms of flu include fever (usually high), headache, extreme tiredness, dry cough, sore throat, runny or stuffy nose, and muscle aches and pains. Gastrointestinal symptoms, such as nausea, vomiting, and diarrhea, are much more common among children than adults. Sometimes the term "stomach flu" is associated with gastrointestinal symptoms. These illnesses usually are not related to influenza viruses.

The primary transmission mode for influenza is person-to-person via respiratory droplets. Droplets from infected persons are released (propelled) outward by coughing or sneezing. Generally, droplet spread is limited to about a meter. Droplets deposit on the nose or mouth of people nearby. Influenza virus also can be spread, although less efficiently, when people touch respiratory deposits on another person or object and then touch their own mouths or noses (or someone else's mouth or nose) before washing their hands.

The incubation period for influenza ranges from one to four days, with the average being two days. When those infected with the virus are contagious depends on their age. Adults are contagious one day prior to becoming ill and then three to seven days after symptoms develop. Children can be infectious for more than 10 days. Young children release virus for less than six days before becoming ill. Usually, more virus is present earlier in an illness than later. Severely immunocompromised persons can remain infectious for weeks or even months.

Complications caused by the flu include bacterial pneumonia and dehydration and worsening of chronic medical conditions. Children may get sinus problems and ear infections. Persons older than 65 years and individuals of any age with chronic medical conditions are at highest risk for serious complications.

By far, the single best way to prevent the flu is for individuals, especially people at high risk for serious complications, to be vaccinated each October or November. Adult immunization usually involves a single injection of an inactivated vaccine (killed virus) in the arm. The vaccine contains three viruses — two types of influenza A virus and one influenza B virus — which are representative of the strains predicted to be dominant within the population that year. The viruses for the flu shot are grown in eggs.

Individuals in close contact with high-risk persons (e.g., health-care workers and household members) should get a flu shot. Consultation with a physician is required if you have ever had a severe allergic reaction to eggs or to a previous flu shot or if you have a history of Guillain-Barré syndrome.

If the match between vaccine viruses and the circulating influenza strains is close, the vaccine prevents influenza in about 70 to 90 percent of healthy individuals under 65. Side effects usually include soreness, redness or swelling at the injection site, low grade fever or aches. The risk of serious harm or death is extremely small.

In 2003, a nasal-spray flu vaccine, LAIV (live attenuated influenza vaccine) was licensed for use. It differs from other licensed products because it contains weakened live influenza viruses instead of killed viruses, and is administered by nasal spray instead of injection. LAIV is approved for use in healthy, nonpregnant persons between five and 49 years. The chances of becoming infected with vaccine virus after close contact with a nasal-spray vaccinated person are low (0.6 to 2.4 percent). The CDC prefers the inactivated vaccine (the flu shot) be used for vaccinating all persons, including health-care workers who have close contact with individuals having weakened immune systems. This is because of a theoretical risk that a vaccine virus could be transmitted and then cause illness.

Influenza information is available at

Dr. Charles John Palenik is an assistant director of Infection Control Research and Services at the Indiana University School of Dentistry. Dr. Palenik has authored numerous articles, book chapters, and monographs, and is the co-author of the popular Infection Control and Management of Hazardous Materials for the Dental Team. He serves on the Executive Board of OSAP, dentistry's resource for infection control and safety. Questions about this article or any infection-control issue may be directed to [email protected].

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