Parents of preschoolers -- especially those already in day care centers or considering group care for the fall -- can now have their toddlers inoculated with a new vaccine against bacterial meningitis.

The vaccine, recommended for all children by the U.S. Public Health Service, would protect children 18 months and older from Haemophilus influenzae Type B, or Hib, the most common cause of bacterial meningitis in young children. Each year, about one out of every 200 children under 5 -- at least 20,000 children in the United States -- will become infected, say spokesmen for the Centers for Disease Control (CDC) and for Praxis Biologics, the Rochester, N.Y., laboratory that developed the vaccine, approved last month by the Food and Drug Administration.

To some 10 percent of those infected, meningitis is fatal. Others develop infections that cause a form of arthritis or other joint diseases, pneumonia, or cellulitis, an inflammation of the tissues under the skin. Sometimes the Hib bacteria settle in the upper respiratory tract and cause a rapid swelling of the epiglottis -- the tissue that covers the opening to the lungs when food is swallowed. In severe cases, this can lead to suffocation.

Twenty-five to 30 percent of those infected with Hib suffer lifelong and debilitating neurologic disorders that "pose a tremendous personal and economic burden on families and the society," says Michael Linnan, a CDC epidemiologist.

"This is an epidemic childhood disease that is now largely preventable, and that's why we are encouraging the vaccine . . . The 20,000 cases reported is most definitely very low because this is not one of the diseases where reporting is required," Linnan says.

Meningitis, an inflammation of the membranes around the brain and spinal cord, begins with a high fever and irritability common in childhood illnesses.

"For a day or two it may look like the flu or a bad cold," says Stephen L. Cochi, medical epidemiologist with the division of bacterial diseases at CDC. "But the child is gravely ill soon after the onset of the symptoms, and this distinguishes meningitis from a harmless upper respiratory infection."

Young children may complain of a stiff neck or show a bulging of the skull's soft spot. "This is not a subtle disease . . . It requires intravenous injections of antibiotics and rapid hospitalization to minimize damage to the brain," Cochi says.

Although many children recover from Hib meningitis, it often causes partial paralysis, blindness, deafness and mental retardation, Linnan says.

The new Hib vaccine, however, is not effective in infants between 6 and 8 months, Cochi says, although children this age have the highest risk of becoming infected. A test trial of an infant vaccine is under way among several thousand native America Eskimo children in Alaska -- another high-risk group -- and a new vaccine for this age group may be available within two years.

In the meantime, the Public Health Service's Advisory Committee on Immunization Practices recommends that all children be vaccinated at 24 months and that children in day-care centers get the single Hib injection at 18 months.

Other candidates for the early vaccination are children with sickle cell disease, those who lost a spleen or were born without one, and infants with malignancies from leukemia or Hodgkin's disease.

For the high-risk infant group, two studies indicate that babies who were predominantly breast-fed under 6 months are less likely to get Hib than bottle-fed babies. These babies apparently benefit from a form of immunity that the mother passes on through her milk. "But there is nothing to show that this protection continues beyond that age even if the mother continues to nurse."

Children 5 and under are most susceptible to the myriad diseases associated with the Hib bacteria because their immune systems are immature. By 6 years of age the immune system is developed and there is a dramatic drop-off in the number of reported cases, Cochi says.

Of the approximately 20,000 cases reported annually of either meningitis or the other related bacterial diseases, 90 percent are among children under 5, according to CDC figures. Seventy-five percent of the Haemophilus bacteria victims are younger than 2 years old, and 25 percent are between 2 and 5.

A test of the vaccine in 98,000 Finnish children showed it was safe and effective. The test was conducted by Praxis Biologics, the company that developed the vaccine, says company spokeswoman Kathy Schafer. Fewer than 1 percent of the 49,000 children inoculated with the active vaccine had significant side effects, such as a fever higher than 101.3 Fahrenheit, Cochi says.

In vaccinated children, "the Hib attack rate was reduced by 90 percent," Schafer says.

Cochi says that because the Praxis vaccine does not contain any protein, it does not cause any untoward side effects.

"It is from the polysaccharide family of vaccines . . . it is made of the outermost coating of the bacterium and of a highly purified and well-defined substance . . . i.e. sugar . . . Only one serious reaction was recorded in the Finnish trials, and that may have been an [allergic] reaction. The child recovered after a shot of adrenalin."

Making the vaccine affordable to all children may be a problem, says Ebenezer Israel, head of the disease control and epidemiology office at the Maryland Department of Health and Mental Hygiene.

At about $6.50 an injection, "the vaccine is as expensive as anything," Israel says. "The immunization program has been cut and is being hit again . . . If we get more state and federal money or assurances that insurance companies will pay for the vaccine, we will implement it quickly for high-risk day care children."