案例分析题

Childhood exposure to a group of chemicals，which are used widely in a variety of products，including water-resistant clothing and microwave popcorn，could reduce the effectiveness of common vaccines，a new study has found.Researchers at the Harvard School of Public Health in Boston looked at over 580children and found that those who had higher concentrations of these perfluorinated（全氟化的）compounds（PFCs）in their blood had lower immune responses to diphtheria（白喉）and tetanus（破伤风）vaccinations.An insufficient immune response to a vaccine can mean a child is actually vulnerable to catching a disease even though they have been vaccinated against it，the researchers said.’When we take our kids to the doctors office to get their shots，we expect that the vaccines are going to work.What we found was that there was an increasing risk that they didnt work if the kids had been exposed to the PFCs，’study author Philippe Grand-jean was quoted as saying by Live Science.

What makes the childrens common vaccines less effective according to the new study？（）