I need information about the administration of vaccines to 3-month-old conjoined twins (joined at the buttocks). For their routine immunization, do we provide one set of vaccinations or two, given that they are conjoined at the buttock but share no major organs?

ACIP does not address this issue. However, CDC recommends that these children should each be vaccinated, notwithstanding they are conjoined. We believe even in conjoined twins who share organs and/or blood supply, vaccination of each child would also be indicated. The rationale is one cannot be sure, even in the latter case, that the common organs/blood supply would eliminate vaccine antigens less quickly, or the immune system(s) would respond adequately, to one dose of each vaccine for the two children. Therefore two doses seems appropriate, that is, one dose of each vaccine for each child.