I think you're thinking of the rotavirus. Research indicates that giving and infant the rotavirus vaccine while they are being breastfed will impede immunological response. So, the researchers have come up with the perfect solution: stop breastfeeding for a few days! (i can't even begin to explain why that 'recommendation' is unbelievably stupid)
Here's an article on it: http://vactruth.com/2010/05/07/vaccine-breastfeeding-immune-response/
Now I have heard it all! BF helps prevents diarrhea and in cases where baby gets it, BF helps treat it. Why a doctor (regardless of some stupid "study") would recommend that we suspend BF, and all the natural antibodies that are part of why its so much better for the baby, so we can inject some man-made version of antibodies is beyond me. My kids (all BF) ddin't even get this shot--my doc said that since I BF it wasn't necessary :-)
Basically, we have half the medical community working like hell to increase awareness about the benefits of BF and getting more women to BF at all, and the other half (this study and a recent one released that refutes the WHO recommendation that all babies be EBF for first 6 mos, that ridiculous "study" said that EBF babies need solids at 4 mos to avoid becoming anemic & suffering brain damage and having food allergies, of course I am paraphrasing) telling us why we shouldn't...
Ridiculous. That's really all I can say...
I was just reading in the book "The Complete Breastfeeding Book of Answers" by Dr. Jack Newman (a leading authority on the subject) that vaccines are actually more effective with breastfeeding, not less. He writes, "Artificial feeding [formula] reduces the effectiveness of vaccinations by diminishing the baby's response and production of the necessary antibodies. Breastfed babies have a better response to vaccinations, and produce more antibodies" (pg. 12).
Hope this helps! :o)