Contrary to some fears, childhood vaccines do not appear to overwhelm the immune system and make youngsters prone to other infections, according to the largest study to examine the issue.
A Danish study found no increased risk for other infectious diseases among more than 800,000 children who received the standard set of vaccinations.
The findings, published in today's issue of the Journal of the American Medical Association, should be reassuring to parents, the researchers said.
"There has been a lot of speculation about this hypothesis -- that if you have a lot of these vaccinations, this could perhaps overwhelm or weaken the child's immune system," said Anders Hviid of the Statens Serum Institute in Copenhagen. "We found no support for that hypothesis."
Other experts said the study should help alleviate concerns about multiple vaccines.
"This clearly provides yet another piece of evidence that supports the safety of routine vaccines for children," said Marie McCormick of the Harvard School of Public Health.
Some parents have become concerned that their children could develop health problems if they received multiple vaccinations in the first few years of life. Most of the controversy has focused on thimerosal, a mercury-containing preservative that had been used in some vaccines and that some researchers and parents suspect may increase the risk for autism.
But a number of public health organizations have found no link between vaccines and an increased risk for autism or other health problems.
Nevertheless, concerns have persisted. Skeptics said the new study does not address the biggest concern about vaccines: that they may increase the risk of developmental problems in children, such as autism or ailments caused by the immune system attacking the body, known as autoimmune diseases.
"It's not infectious diseases that parents are concerned about," said Barbara Loe Fisher of the National Vaccine Information Center, a nonprofit advocacy group based in Vienna. "They are concerned about learning disabilities and autism and asthma and diabetes."
Walter Orenstein, associate director of the Vaccine Center at Emory University in Atlanta, said the study is important because it was the largest to address the issue. "This is a very reassuring study," he said.
Hviid and his colleagues studied data collected on 805,206 children born in Denmark between 1990 and 2001 for their first five years of life, examining whether any of six standard vaccines children receive increased the risk for seven other major infectious diseases.
The vaccines were Haemophilus influenzae Type B, diphtheria-tetanus-inactivated poliovirus, diphtheria-tetanus-acellular pertussis-inactivated poliovirus, whole-cell pertussis, measles-mumps-rubella, and oral poliovirus. They found no significant increased risk of being hospitalized with the other infections: viral and bacterial pneumonia, bloodstream infections known as septicemia, meningitis, diarrhea, upper respiratory infections, and viral central nervous system infections.
"This is another study that should allay fears about vaccine safety," Hviid said.
Although the timing of childhood vaccinations in Denmark is slightly different than in the United States, the vaccines are the same.