What is the effectiveness of one dose of Measles, Mumps, and Rubella (MMR) vaccine versus two doses?

Effectiveness of One Versus Two Measles Vaccines

Two doses of MMR vaccine are significantly more effective than one dose, with approximately 5% of children failing to develop immunity after a single dose, while two doses provide near-complete protection against measles, mumps, and rubella. 1

Effectiveness Data

One-Dose Effectiveness

• Approximately 95% of children develop immunity to measles after the first dose 1
• For mumps and rubella, first-dose response rates exceed 95% 2
• Measles response rates after the first dose vary by age of administration:
  • 73% when given at 9 months
  • 88% when given at 11 months
  • 90% when given at 12 months 2

Two-Dose Effectiveness

• Almost all persons who do not respond to the first dose will develop immunity after the second dose 1
• Two-dose seroprotection rates at 10-year follow-up:
  • Measles: 93.7%
  • Mumps: 73.4%
  • Rubella: 83.9% 3
• The second dose is not considered a booster but completes the primary immunization series 4

Rationale for Two-Dose Schedule

The two-dose schedule is recommended primarily for these reasons:

1. Primary vaccine failure: The second dose provides immunity to the small proportion of individuals who fail to respond to the first dose 1

2. School-based outbreaks: Two doses help prevent outbreaks in school settings, even when first-dose coverage exceeds 95% 5

3. Elimination goals: Complete two-dose coverage is essential for measles elimination efforts nationally and globally 5

4. Waning immunity: While not the primary reason for the second dose, antibody levels do decline over time, particularly for mumps 3

Timing of Doses

• First dose: Recommended at 12-15 months of age 1
• Second dose: Recommended at 4-6 years of age (before kindergarten entry) 1
• Minimum interval between doses: 28 days, though 3 months is typically recommended for children under 13 years 1

Special Considerations

• High-risk areas: In areas with high measles risk, the first dose may be given at 12 months rather than waiting until 15 months 1
• Catch-up vaccination: Unvaccinated children and adolescents should receive two doses separated by at least 28 days 1
• International travel: Infants 6-11 months should receive one dose before travel, but this dose doesn't count toward the routine two-dose schedule 1

Clinical Implications

• The two-dose MMR schedule has dramatically reduced the incidence of measles, mumps, and rubella, leading to elimination of endemic measles in 2000 and rubella in 2004 4
• Despite waning antibody levels, protection against disease is largely retained long-term after two doses 3
• Mumps antibodies wane more quickly than measles and rubella antibodies in both two-dose and three-dose recipients 3

Pitfalls and Caveats

• A common misconception is that the second dose is a "booster" - it's actually to provide immunity to those who didn't respond to the first dose 4
• Delaying the second dose increases vulnerability to outbreaks, particularly in school settings 5
• Vaccine effectiveness can be compromised by improper storage - MMR must be stored at 2-8°C and protected from light 1
• During outbreaks, additional doses may be recommended regardless of vaccination history 4

The evidence clearly demonstrates that the two-dose MMR schedule provides superior protection compared to a single dose, with the second dose playing a crucial role in achieving population immunity sufficient for disease elimination.