When a shot is not: PCBs may impair vaccine-induced immunity.

Exposure to certain pollutants early in life may do lasting harm to the immune system by blocking its response to vaccinations, suggests a study from the Faroe Islands.

That archipelago, which lies in the North Atlantic between Scotland and Iceland, makes a unique laboratory for studying the health effects of chemicals called polychlorinated biphenyls (PCBs). The Faroese traditionally hunted pilot whales, and some still eat the animals' blubber, which is heavily contaminated with PCBs. Those organic pollutants linger for years in body fat and are passed from mother to child during pregnancy and through breast-feeding.

"Some Faroese have 100-fold higher exposures to PCBs than others," says Philippe Grandjean of the Harvard School of Public Health in Boston and the University of Southern Denmark in Odense.

Along with four colleagues in Denmark, Grandjean studied several hundred mothers and their children on the islands. The researchers measured concentrations of PCBs in each mother's blood and milk around the time she gave birth and in the children's blood at either 18 months or 7 years of age.

The scientists also tested the children's blood for antibodies that protect against tetanus and diphtheria. All the children had been vaccinated against both diseases.

The researchers chose to study immune responses to those particular vaccinations because several important cell types are involved in creating antibodies against the diseases. "We are essentially looking at the integrity of the whole immune system," Grandjean says.

He and his colleagues linked high PCB concentrations in mothers' blood to low tetanus-antibody concentrations in their 7-year-olds. Similarly, concentrations of PCBs and diphtheria antibodies were inversely correlated in blood from 18-month-olds, the team reports in the August PLoS Medicine. Data on both antibody types suggest that immunity is influenced by PCB exposure both in utero and during infancy, presumably from breast-feeding.

Judging from the 7-year-olds' blood concentrations of diphtheria-fighting antibodies, about "25 percent of the kids were not protected" by their vaccinations, Grandjean says.

Other persistent organic pollutants could similarly weaken reactions to vaccines, Grandjean says. Animal studies have indicated that dioxins, pollutants that chemically resemble certain PCBs, affect the developing immune system. Such pollutants could also more generally limit people's ability to fight off infections, whether or not they've been vaccinated against them, Grandjean says. Recent studies in Canada found that ear and respiratory infections are more frequent in children with greater-than-average PCB exposures.

The new study appears to be the first to link PCB exposure to an immunological outcome of clinical importance, says developmental psychologist Joseph Jacobson of Wayne State University School of Medicine in Detroit. He and other researchers have reported that PCBs harm cognitive maturation in children.

The new study suggests that limiting breast-feeding might ameliorate some of the effect that PCBs have on immunity, but Grandjean stresses that it's not wise to discourage breast-feeding, even in such a heavily PCB-exposed population as the Faroese. "I certainly wouldn't make any clinical recommendations about breast-feeding based on this data," Jacobson says.