Thank you for your inquiry.
It has always been my assumption that this admonition was related to the fact that the immunoglobulin employed for replacement therapy would be expected to have antibodies against live viruses if taken from people who were immune. Since it employs pooled sera, it is highly probable that such antibodies would be contained within the replacement immunoglobulin.
As you know, the half-life of IgG is approximately 23 days (depending on subtype measured). We know also that the clearance of drug from circulation usually takes five half-lives. Thus, one would expect that the vast majority of replacement immunoglobulin would be gone at the end of six months. I have always seen that figure, rather than a year, listed as the time required after cessation of immunoglobulin replacement to permit immunization with live attenuated viral vaccines. However, I assume that the full year noted in your inquiry is simply to gain more assurance that interference wold not occur.
You will find a brief discussion of this on the CDC website here and also in the article which is abstracted below.
Thank you again for your inquiry and we hope this response is helpful to you.
Interference of immune globulin with measles and rubella immunization
The Journal of Pediatrics
Volume 122, Issue 2 , Pages 204-211, February 1993
Passively acquired antibody may interfere with the active antibody response to live viral vaccines such as measles and rubella. To evaluate the duration of this inhibitory effect, we measured the measles and rubella antibody responses of Apache children immunized with measies, mumps, and rubella vaccine at varying intervals after administration of an immune globulin termed bacterial polysaccharide immune globulin (BPIG). This specific immune globulin contained measles and rubella antibody titers similar to those in standard intramuscularly and intravenously administered immune globulins. Antibody responses to measies vaccine were inhibited for up to 5 months after a BPIG dose of 80 mg IgG perkllogram of body weight, but responses to rubella vaccine were inhibited for only 2 months. Most children who had a decreased measles antibody response to primary measles, mumps, and rubella immunization given 11/2 to 4 months after BPIG administration responded to a booster immunization given 6 months after their last BPIG dose. We conclude that high doses of immune globulin (>10 mg/kg) may inhibit the antibody response to measles for more than 3 months. We propose that the interval between administration of immune globulin and measles and rubella immunization be adjusted on the basis of the dose of immune globulin.
Phil Lieberman, M.D.