Which vaccine should be avoided during the postpartum period for breastfeeding mothers?

During the postpartum period, breastfeeding mothers should avoid the smallpox vaccine. This vaccine contains a live virus that can pose a risk to both the mother and the breastfed infant. Although the risk of transmission through breastfeeding is low, the potential complications arising from the live virus make it advisable for nursing mothers to refrain from receiving this vaccine.

In contrast, the MMR (measles, mumps, rubella) vaccine and the varicella (chickenpox) vaccine are both live vaccines but are considered safe for breastfeeding mothers. This is because the live attenuated viruses in these vaccines do not seem to pose a risk to an infant through breast milk. The tetanus toxoid vaccine is an inactivated vaccine and is also safe during breastfeeding, so it can be administered without concern.

Thus, the smallpox vaccine is the only one among the listed options that should be avoided by breastfeeding mothers due to the associated risks with its live viral content.