In a survey of more than 2,000 people, the Centers for Disease Control and Prevention found four different PFCs in the blood serum of nearly all of them.
The Harvard scientists, led by Philippe Grandjean, of the school's department of environmental health, decided to study 656 children born in the Faeroe Islands - in the Norwegian Sea between Scotland and Iceland - because the people there eat a lot of fish known to have lots of PFCs.
They looked at prenatal and postnatal exposure and then measured how well the diphtheria and tetanus vaccines worked at ages 5 and 7.
They concluded that when the postnatal exposure doubled, the "antibody concentration" in the child, an indication of the vaccines' effectiveness, was halved.
This was merely an association, not a cause, but "we believe that we have very strong evidence that there is something here that we need to be aware of," said Grandjean, a physician who is also associated with the University of Southern Denmark.
"Some of these kids had been vaccinated four times, and at age 7, they weren't even protected," he said. "This is mind-boggling."
No one is really worried about diphtheria and tetanus, as such, because they're so rare. But the vaccines are markers of the immune system's response to vaccines.
So even more worrisome, Grandjean said, is the possibility that the children's immune systems overall are sluggish.
The study was published in the Jan. 25 issue of the Journal of the American Medical Association.
Not everyone is rushing to ditch the microwave popcorn.
Paul Offit, who directs the Vaccine Education Center at Children's Hospital of Philadelphia, said he was disappointed in the study.
