Babies get their first measles shot at 12 months old, with a second dose at 4 to 6 years

The measles vaccine is given as part of the MMR shot, which protects against measles, mumps, and rubella. Infants under 12 months are not routinely vaccinated because maternal antibodies — protection passed from the mother during pregnancy — still circulate in their blood and would interfere with the vaccine's effectiveness. Once those antibodies fade, usually around 12 months, the vaccine can work as intended.

The second MMR dose comes later in childhood, typically between ages 4 and 6, though it can be given as soon as 28 days after the first dose if needed. This two-dose schedule ensures that children who did not develop full immunity from the first shot are protected by the second.

Key Takeaways

  • Babies receive their first MMR vaccine at 12 months of age, when maternal antibodies no longer block the vaccine from working.
  • A second MMR dose is given between ages 4 and 6 to catch anyone who did not develop immunity from the first shot.
  • Infants under 12 months can still catch measles, which is why keeping unvaccinated people away from babies is important.
  • If a baby is exposed to measles before 12 months, the doctor may give a temporary protective shot called immune globulin.

Why babies under 12 months don't get the vaccine yet

A newborn's immune system is not yet mature enough to mount a strong response to most vaccines. Instead, babies rely on maternal antibodies — proteins the mother passes through the placenta during pregnancy and through breast milk afterward. These antibodies provide temporary protection against diseases the mother has immunity to, including measles.

The problem is that these same maternal antibodies also neutralize the measles vaccine before it can teach the baby's own immune system to recognize the virus. Vaccinating before 12 months would be largely wasted — the vaccine would be blocked, and the baby would not develop lasting protection. By 12 months, maternal antibodies have faded enough that the vaccine can work, but the baby is still protected during that window.

This timing is not arbitrary. Public health agencies including the CDC determined that 12 months is when the balance tips: maternal antibodies are low enough for the vaccine to be effective, but the risk of measles exposure is still manageable for most babies in vaccinated communities.

What happens if a baby under 12 months is exposed to measles

Measles is highly contagious — about 9 out of 10 unvaccinated people who are exposed will catch it. A baby under 12 months who is exposed to measles faces real risk, because they have no vaccine-based immunity yet and rely only on whatever maternal antibodies remain.

If exposure occurs, a doctor may give the baby immune globulin, a shot containing antibodies from vaccinated donors. This provides temporary protection and can prevent measles or reduce its severity if given within 6 days of exposure. It is not a permanent solution — the baby will still need the MMR vaccine at 12 months — but it can be lifesaving in the immediate situation.

This is why pediatricians ask about measles exposure at visits and why keeping sick people away from unvaccinated infants matters. In communities where measles is circulating, the risk to babies under 12 months is real.

The two-dose schedule and why both shots matter

The first MMR dose at 12 months protects about 93 percent of babies who receive it. The remaining 7 percent do not develop immunity from that single shot — their immune systems simply did not respond strongly enough. The second dose, given at 4 to 6 years, catches most of those children and brings overall protection to about 97 percent.

This is why both doses are necessary. A child who receives only one MMR shot has a meaningful gap in protection. Public health agencies recommend the two-dose schedule because it accounts for the fact that immunity is not universal after a single exposure, even a vaccine exposure.

In some situations — such as during a measles outbreak or before international travel — the second dose can be given as soon as 28 days after the first. But the standard schedule spaces them out over years because there is no medical reason to rush, and spacing allows time to monitor for any rare side effects.

Babies born to mothers without measles immunity

A baby's maternal antibodies are only as strong as the mother's own immunity. If the mother was vaccinated against measles, she passes protective antibodies to the baby. If she was not vaccinated and has no natural immunity to measles, the baby receives no protection from her.

This matters because it means some babies are born with no measles protection at all. These infants are entirely dependent on the people around them being vaccinated. If an unvaccinated person brings measles into the home, these babies have no defense until they can receive their own vaccine at 12 months.

Mothers who are unsure of their own measles immunity can ask their doctor for a blood test. If they have no immunity, they can be vaccinated before pregnancy or immediately after delivery (though not during pregnancy, as MMR is a live vaccine). This protects both the mother and the baby she carries or breastfeeds.

Side effects and safety of the MMR vaccine in babies

The MMR vaccine is a live vaccine, meaning it contains a weakened form of the measles, mumps, and rubella viruses. It cannot cause measles, mumps, or rubella — the viruses are too weak — but it does trigger an immune response that teaches the body to recognize and fight the real diseases.

Common side effects in babies and young children include fever, rash, and swollen glands, usually appearing 7 to 12 days after the shot. These are signs that the immune system is responding and are not dangerous. Serious side effects are rare — severe allergic reactions occur in fewer than 1 per million doses.

The vaccine has been used for decades and studied extensively. The CDC and other health agencies continue to monitor its safety. The risks of measles itself — including pneumonia, brain inflammation, and death — far outweigh the small risks of the vaccine.

Frequently Asked Questions

Can a baby get the measles vaccine before 12 months if there is an outbreak?

In rare cases of active measles transmission, a doctor may give an early dose at 6 to 11 months. This early dose does not count as the first dose of the two-dose series — the baby will still need two standard doses starting at 12 months. Early vaccination is used only when the risk of measles exposure is very high.

What if my baby was premature — does the 12-month timing still apply?

The 12-month mark is based on age since birth, not adjusted age. A baby born at 28 weeks gestation receives the MMR vaccine at 12 months after birth, not 12 months after the due date. Your pediatrician will confirm the right timing at your visits.

Is the MMR vaccine safe if my baby has a fever or mild illness?

A mild illness like a cold does not delay vaccination. Babies can safely receive the MMR vaccine while they have a low fever or minor respiratory symptoms. A moderate to high fever or more serious illness may prompt your doctor to reschedule, but mild illness is not a reason to wait.

Can breastfeeding babies get the MMR vaccine at 12 months?

Yes. Breast milk contains some maternal antibodies, but by 12 months they have faded enough that the vaccine will work. Breastfeeding does not interfere with the MMR vaccine and can continue without interruption after vaccination.

What if I missed the 12-month appointment — when should my baby get vaccinated?

The MMR vaccine can be given at any age after 12 months. If your baby is older than 12 months and has not yet been vaccinated, contact your pediatrician to schedule it as soon as possible. The sooner the first dose is given, the sooner protection begins.