One dose of the MMR vaccine is very effective, but not as protective as the full two‑dose series. Health agencies therefore recommend two doses for best and more durable protection.

Protection after one dose

• For measles, one MMR dose prevents about 93–95% of cases in vaccinated people.

• For mumps, one dose is less strong, preventing roughly 70–80% of cases.

• For rubella, one dose is highly protective, around 89–97% effective.

Why a second dose is recommended

• A small percentage of people do not respond to the first dose (primary vaccine failure), and the second dose “catches” most of them, pushing measles protection close to 97% and mumps into the mid‑80s or higher.

• Two doses also help maintain strong population‑level (herd) immunity, which is important because measles is extremely contagious and outbreaks can occur if coverage or individual protection is incomplete.

In short, one dose gives substantial protection, especially against measles and rubella, but completing the two‑dose series is important for optimal and longer‑lasting protection.