Prevnar, a vaccine that is dramatically decreasing the number of American babies' infections from a common but serious bacterium, also increases the number of other infections the babies get, according to an Israeli researcher. No other vaccine in history has shown this unintended effect, said Ron Dagan, director of the Pediatric Infectious Disease Unit at Soroka University Medical Center in Beer-Sheva, Israel.

"[Its importance] is not minor," he said, "but for the moment, we cannot estimate totally the clinical effect because we still do not have enough vaccinated people."

Hypothetically, Prevnar's disease-enabling ability might someday become as powerful as its disease-preventing capacity, rendering it effectively useless, said Dagan, and necessitating a new vaccine design. Future vaccines utilizing the same approach of targeting specific strains in a larger microbial family might also create this effect, according to other researchers.

Prevnar, a 7-valent pneumococcal conjugate vaccine marketed by Wyeth, helps prevent...

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