In a study spanning the United States, Europe, the Middle East and Asia, researchers writing in the Feb. 15 New England Journal of Medicine say a nasal spray flu vaccine reduced the influenza "attack rate" in children by 55 percent when compared with a group of children who received the traditional flu shot in the arm or thigh.

"Children get the flu twice as often as adults," said Robert Belshe, M.D., a vaccine researcher at Saint Louis University School of Medicine and the lead author of the study. "It's important to vaccinate kids against influenza -- and to identify new and more effective flu vaccine options -- because kids have a higher attack rate for influenza infection than adults. And children, when they bring the illness home, tend to pass it on to adults in their household."

The attack rate is the number of people who get sick as measured against the total number of people in a particular study population.

Belshe said the study points to new thinking about how to best protect very young children from influenza and offers insight into the safety and efficacy of influenza vaccines for children, whether administered through the nose or with a needle.

The study published in the New England Journal of Medicine was conducted in 8,475 children between the ages of 6 months and 5 years at 249 sites in 16 countries and was funded by MedImmune, Inc. It is the largest ever pediatric study comparing a nasal spray flu vaccine head-on with the traditional flu shot. About half of the children in the study received an FDA-approved injectable flu vaccine (which contains an inactivated flu virus), and the other half received a refrigerated version of the nasal spray influenza vaccine. The nasal spray influenza vaccine FluMist© (Influenza Virus Vaccine Live, Intranasal) is currently FDA-approved for use in healthy people between 5 years and 49 years.

"We compared the attack rate of the flu in children who were randomized to receive either the traditional shot or the FluMist nasal spray -- for the entire 2004-2005 winter season," Belshe said. "In this worldwide trial, the nasal spray flu vaccine reduced culture-confirmed influenza disease by 55 percent compared to the flu shot."

Belshe said the traditional flu shot and the nasal spray flu vaccine stimulate different types of immune responses. Whereas the flu shot stimulates antibodies in the blood, the nasal spray vaccine stimulates antibodies both in the bloodstream and in the nose, and may stimulate a cellular immune response as well.

"The big difference is the induction of antibodies in the nose, which is important because this is where the flu virus usually enters the body. The nasal spray flu vaccine appears to induce a more complete immune response," Belshe said.

Researchers Assess Safety Data

This large study -- which will include an article in the New England Journal of Medicine and additional data on the journal's Web site -- also points to important safety observations regarding the use of an intranasal flu vaccine. Among the conclusions from this study:

FluMist was well-tolerated in children older than 12 months, provided they didn't have a past medical history of wheezing.

For children older than 12 months who did have a previous history of wheezing, the FluMist group was associated with a higher rate of hospitalization from all causes in the six months following the vaccination. This trend was not statistically significant.

The nasal spray group of children younger than 12 months had more hospitalizations in the six months following vaccination than children in the injectable vaccine group. Belshe said that these final findings demonstrate the need for an improved flu vaccine for infants younger than 12 months.

Fewer Ear Infections Reported With Flumist, And Broader Protection Against Mismatched Strains

In another study finding, Belshe said the group that received FluMist had significantly fewer ear infections associated with influenza than did the group that received the injectable shot.

Ear infections can be a complication of the flu because the illness damages the mucus membranes of the sinuses, ears and trachea, he explained.

Belshe said that many people do not realize just how serious a disease influenza is and how the illness can be potentially deadly in the elderly.

"Some cases of the flu are mild, but many cases are not mild," he said. "More than 30,000 adults die each winter because of influenza and its complications."

Belshe said that influenza vaccine must be manufactured several months before the flu season, so in some years there's a vaccine "mismatch" with the strains of flu that are circulating. The study results also indicated that this nasal spray vaccine prevented 58 percent more cases of flu when compared to the flu shot even when the circulating strains of influenza were "mismatched" with those contained in the seasonal vaccine.

"We found that the nasal spray flu vaccine was good at preventing illness even when the vaccine was mismatched," he said.

Study participants were randomized to receive either FluMist or the injectable influenza vaccine during the 2004-2005 influenza season. Each participant received a placebo nasal spray or placebo injection to preserve the double-blind design of the study. Participants also were followed through the influenza season and evaluated to identify illnesses caused by influenza virus. The trial included a record 6,384 previously unvaccinated children, with nearly 50 percent of those children younger than 2 years of age.

This article has been adapted from a news release issued by Saint Louis University.