Final answer:
Live virus vaccines are contraindicated in pregnancy due to the risk of crossing the placenta and infecting the fetus, potentially leading to serious birth defects or miscarriage. They are not recommended for pregnant women because of the immune suppression during pregnancy and the risk of back mutations that can lead to an epidemic.
Step-by-step explanation:
Live virus vaccines are contraindicated during pregnancy because of the risk that the attenuated virus can cross the placenta, infect the fetus, and cause severe birth defects or miscarriage. This is particularly concerning because the immune system is downregulated during pregnancy, increasing the vulnerability of the fetus to infections. For example, if a pregnant woman contracts rubella, also known as German measles, particularly during the first trimester, it can lead to congenital rubella syndrome in the newborn, resulting in a high incidence of stillbirth, spontaneous abortion, or congenital defects. Therefore, live vaccines such as those for measles, mumps, and rubella (MMR) are not recommended for pregnant women. Additionally, any exposure to or infections caused by live vaccine strains can be dangerous as there is a low but significant risk that the vaccine viruses undergo back mutations and revert to a disease-causing form, which could lead to an epidemic.
A conjugated vaccine is necessary in some cases to prevent the aforementioned risks associated with live vaccines. These vaccines use pieces of the germ like its sugar or protein, which are linked to carrier proteins, to provoke an immune response without the risk of causing the disease itself.