The same vaccine may protect some children more than others, but it's not known why. Researchers at Harvard University School of Public Health suspected that exposure to toxins could damage a baby's immune system and lessen a vaccine's effectiveness.

They studied children who had been exposed to polychlorinated biphenyls or PCBs. These persistent organic chemicals are now banned under an international treaty, however they continue to leak into waterways from old electrical equipment once used in industry.

PCB levels are especially high among people in the Faroe Islands of the North Atlantic who eat a traditional diet of whale blubber. PCBs - which are concentrated in fatty tissue like blubber - pass to babies through their mother's breast milk.

The study looked at two groups of Faroese children -- at age 18 months and 7 years old -- who had received routine vaccinations for diphtheria and tetanus. Researchers measured levels of both protective antibodies and toxins in each child. Lead author Philippe Grandjean says the findings indicate that PCBs can, indeed, interfere with vaccine effectiveness. "At age 18 months we saw a very clear association between the PCB exposure and the diphtheria antibody levels. The effectiveness decreased 20% for each doubling of the [PCB] exposure." Pinkerton adds, "This is a substantial decrease. However, at age 7, diphtheria [protection] had become so low that 25% of the children no longer had protective concentrations of the antibodies in their serum [blood]." The results were not as dramatic for tetanus, although researchers still saw an effect.

Grandjean suggests that if PCBs can compromise a child's immune system, other toxins might, as well. "Perhaps pollutants could be part of the reason that children are not reacting that well, that the vaccines are not [having their intended effect]."

Phillipe Grandjean says increased exposure to toxins could threaten the ability of vaccines to do their job, impacting public health.The study is published in the online edition of the Public Library of Science Medicine.