Q. My 2-month-old daughter recently had her first set of immunization shots. She got a new one, or at least one our 7-year-old son didn't get when he was a baby. It was called an Hib vaccine. What is this immunization supposed to protect against?

A. Hib stands for Haemophilus influenzae, type b. H. influenzae, also called "H flu," for short, is a bacterium that can cause serious infections in young children. Although their names sound alike, the H flu bacterium is unrelated to the influenza "flu" virus. In addition, H flu infections are quite different from the common flu infection. The Hib shot, the newest addition to childhood immunizations, is designed to prevent Hib infection.

H flu is the leading cause of bacterial meningitis in children. Meningitis is a potentially life-threatening infection of the brain's covering. Even with good treatment, about 5 percent of children with H flu meningitis die, and one in three suffer some permanent brain damage.

Besides meningitis, H flu can cause pneumonia, infections of the blood, such as sepsis, or blood poisoning; skin or joint infections; and epiglottitis, an infection seen mostly in young children.

The epiglottis is a flap of tissue that keeps food and drink from going down the windpipe when you swallow. If it gets infected, it can swell and close off the breathing passage. Symptoms of epiglottitis are fever, sore throat, trouble swallowing, drooling and a croup-like, barking cough.

Hib vaccines first became available in 1985, but they didn't work well in children under 2, the age group at highest risk of Hib infection. Recent improvements have made them effective in young children, and doctors now start Hib shots at 2 months.

Because of improvements in the vaccine, the Hib immunization schedule has changed several times recently. To make things more confusing, several different Hib vaccines are available, and each has a different schedule of administration.

Currently, PedvaxHIB is given at 2, 4 and 12 months of age. HibTITER is given at 2, 4, 6 and 15 months. ProHIBit only works in children 15 months and older. Newborns should receive one of the vaccines that works at 2 months of age.

What if your child didn't get the improved Hib vaccine at 2 months of age? Children older than 2 months can still get a series of Hib shots to complete their vaccination against this disease. Children older than 15 months and younger than 5 years need only one shot to complete their immunization.

Children older than 5 who have never received an Hib shot generally don't need one. Most children over 5 have usually already been exposed to the H flu germ and have developed a natural immunity to it.

If you have children under 5 who didn't get the new Hib shot when they were younger, check with your doctor about their getting this latest preventive health vaccine.

Jay Siwek, a family physician from Georgetown University, practices at the Fort Lincoln Family Medicine Center and Providence Hospital in Northeast Washington.

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