It sounds like a dream come true for parents—a vaccine that can help prevent agonizing middle ear infections in infants and toddlers.

A new study found that the pneumococcal conjugate vaccine (PCV7) moderately lowered the incidence of middle ear infections and reduced the need for pressure-equalizing ear tubes, a common surgical treatment for recurrent infections.

"Children prone to recurrent ear infections benefit from the vaccine," says lead study author Katherine A. Poehling, an assistant professor of pediatrics at Monroe Carrell Jr. Children's Hospital at Vanderbilt University in Nashville, Tenn.

But do not expect this scourge of childhood, which affects more than 80 percent of children under age three and costs $5 billion to treat each year, to go the way of whooping cough anytime soon. The reason: PCV7 prevents only a fraction of the infections that cause it. According to the Centers for Disease Control and Prevention (CDC), the vaccine combats the seven most common strains of pneumococcal bacteria behind otitis media (middle ear infection). But pneumococcal bacteria cause only 30 to 55 percent of such infections; other classes of bacteria cause the rest.

The PCV7 vaccine has been part of the routine immunization schedule for infants in the U.S. since 2000. It was developed to prevent meningitis, invasive pneumonia and other potentially deadly diseases. It is given to infants in four doses: at two, four, six and 12 to 15 months.

The findings, published in the April issue of Pediatrics are the first to document the long-term effects of the vaccine on middle ear infections.

Study co-author J. Pekka Nuorti, a medical epidemiologist at the CDC's National Center for Immunization and Respiratory Diseases, estimates that the PCV7 vaccine prevents two million cases of otitis media in children under two years of age in the U.S. annually.

"Studies show that the number of middle ear infections caused by the pneumococcus serotypes in the vaccine have declined dramatically, but others have increased by 30 percent," Nuorti says. Bacteria on the rise may be filling the ecological void left by the decline in the vaccine's seven pneumococcus strains. But, he adds, two vaccines in the pipeline would cover more pneumococcal strains, including those whose rates of occurrence are escalating.

Even so, it is unlikely that any vaccine will relegate middle ear infections to the history books.

"Parents shouldn't think that just because their kids are getting vaccinated that they won't get otitis media," says Richard Rosenfeld, director of otolaryngology at Long Island College Hospital in New York City. "Seventy percent to 80 percent of the tendency to get otitis media is genetic. Beyond that are a host of factors, including smoking in the household, attending day care and breast-feeding for the first three months of life…. The vaccine alters that equation, but the effect isn't dramatic."