A new vaccine against Hemophilus influenza type b (Hib), the leading invasive bacterial disease in children, has proved to stimulate an immune response in infants as young as 2 months old. The Hib vaccine now on the market is effective only in children older than 2, by which time the highest risk period for Hib infection has passed.

Scientists at the Children's Hospital in St. Louis and the Batavia (Illinois) Medical Center report in the current issue of Pediatrics that 50 vaccinated infants, ages 2 to 6 months, were shown to produce antibodies when injected with an Hib vaccine plus one booster shot. The antibody response was stronger when the booster shot was given two months, rather than one month, after the first.

"We hope the antibodies induced by the vaccine will be protective," said Dr. Allan A. Lenoir, now at Children's Hospital of Miami. "But the only way we can know for sure that a vaccine will protect is to follow a large number of children who have been vaccinated over a several year period."

Because 80 percent of Hib victims are younger than 2, the current vaccine has been insufficient to prevent the spread of Hemophilus type b and its serious complications, which have a fatality rate of about 5 percent.

Hemophilus influenzae, despite its name, does not cause the flu. The type b agent causes only invasive disease, including bacterial meningitis (an infection of the brain that causes permanent brain damage in about one out of four survivors), epiglottitis (inflammation of the flap over the voice box and windpipe, which in its most severe form could actually choke the child), septicemia (infection of the blood) and septic arthritis (a severe infection of the joints).

The Centers for Disease Control says that invasive Hemophilus type b infection occurs in 20,000 children a year, and accounts for 800 to 1,000 deaths annually. Roughly half of these cases, and half of these deaths, are from meningitis alone.

The new vaccine differs from the licensed vaccine by linking the Hemophilus bacterium, which on its own is unrecognizable to the immature immune system, to a different bacterium to which infants are able to respond. According to Lenoir, this is the first vaccine to show an antibody response in a signficant number of children after just one injection. He says the level of antibody responses in young children given his vaccine is comparable to that seen in older children who have been given the licensed vaccine and who were shown to have been protected from disease.

A large-scale efficacy study, with results expected in five years or more, must be done before the vaccine can be marketed. Meanwhile, clinical trials are under way in Finland and the U.S. on a similar, competitive vaccine. Dr. Joel Ward, an associate professor of pediatrics at the University of California at Los Angeles, is testing the efficacy of this second Hib vaccine, in which the agent is linked to diphtheria toxoid, in 2,000 Eskimo and Indian babies in Alaska, where the rate of Hib infection is about 20 times the national rate. Ward says the result of the three-year study should be available next year.