Why are some vaccines delayed until 12 months, such as MMR?

• A. Febrile seizures are more likely in younger infants with some vaccines.
• B. Maternal antibodies neutralize some vaccines and are delayed until 12 months.
• C. The risk of adverse effects is lower for some vaccines after the first year.
• D. Too many vaccines at once can overwhelm the infant's immune system.

Answer: (B)

Maternal antibodies that the infant receives at birth can interfere with certain vaccines, especially live vaccines like MMR. These antibodies can neutralize the vaccine viruses before the infant’s own immune system can respond, making the vaccine less effective. By about 12 months, the levels of those maternal antibodies have waned enough that the MMR vaccine can replicate safely enough to elicit a good immune response and provide long-lasting protection. That’s why the first dose of MMR is scheduled around 12 months.