Children wince at the numerous vaccine needlepricks they endure. Recently their parents have been wincing, too, at a related worry: that all those vaccines might overwhelm a child's immune system, causing illness instead of preventing it.

But in this week's Journal of the American Medical Association, researchers offer both reassuring words and numbers. They examined the records of about 800,000 children and found no connection between more vaccines and more "nontargeted infectious diseases," or illnesses the vaccinations don't cover. This was also true for shots that target multiple diseases, such as the measles-mumps-rubella (MMR) vaccine.

Researchers analyzed the vaccination history of children born in Denmark from 1990 to 2001 and then looked at their hospitalizations for infectious diseases, including pneumonia, bacterial meningitis, diarrhea, and acute upper respiratory infections. They explored associations between a total of six childhood immunizations and seven infectious diseases.

The authors noted that, rather than adding risk for nontargeted diseases, the vaccines actually appeared to protect against them a lot of the time. For example, the MMR vaccine appeared to offer some protection from viral pneumonia and the diptheria-tetanus-pertussis-polio (DtaP-IPV) vaccine reduced the risk of bacterial meningitis. All in all, there were 15 of these protective connections between vaccines and illnesses.

The scientists did find a slightly raised risk of acute upper respiratory infections among kids who had immunizations for Haemophilus influenzae type B. (This is not the flu virus but a type of bacteria that may cause several diseases.) But, say the researchers, this was likely due to chance, not cause and effect. Otherwise, the infection risk would have increased with each vaccine dose given, but it did not. Nor was risk of infection higher close to the time when the shots were given.

The reassurance comes as no surprise to Paul Offit, chief of infectious diseases at the Children's Hospital of Philadelphia. He emphasizes that the immunological challenges vaccines present are a "drop in the ocean" in comparison with those presented by the 100 trillion bacteria he estimates inhabit the surface of a human body. The study underlines how important vaccines really are, he adds.

"If you look at the numbers of lives that are saved by vaccines," says Offit, "it's millions of lives each year."

For more information: The American Academy of Pediatrics has a page on immunizations, with a question-and-answer section for parents.

The Centers for Disease Control and Prevention publishes the current vaccination schedule.