Chances of Contracting Measles After MMR Vaccination
The MMR vaccine is highly effective at preventing measles, with a 95% protection rate after one dose and 96% after two doses, meaning the chance of getting measles after proper vaccination is approximately 4-5% after one dose and 3-4% after two doses. 1
Vaccine Effectiveness Against Measles
The protection provided by MMR vaccination is substantial and well-documented:
- After one dose: 95% effective at preventing measles (RR 0.05,95% CI 0.02 to 0.13), leaving approximately a 5% chance of infection if exposed 1
- After two doses: 96% effective at preventing measles (RR 0.04,95% CI 0.01 to 0.28), leaving approximately a 4% chance of infection if exposed 1
- Post-exposure prophylaxis: Even when given after measles exposure, at least one dose provides 74% protection (RR 0.26,95% CI 0.14 to 0.50) 1
Long-Term Antibody Protection
While antibody levels decline over time, protection against disease remains largely intact:
- At 10 years post-vaccination: 93.7% of two-dose recipients maintained protective antibody levels against measles 2
- Antibody waning: Measles neutralizing antibodies decline annually, but the pattern and magnitude vary by individual baseline response 2
- Clinical significance: Even when antibodies wane, protection against actual measles disease is largely retained, unlike mumps where waning is more clinically significant 2
Vaccine-Related Measles-Like Symptoms vs. True Infection
A critical distinction must be made between vaccine-related symptoms and actual measles infection:
- Vaccine-induced rash: Approximately 5% of vaccinated persons develop a transient measles-like rash 7-10 days after vaccination 3
- Timing: These symptoms typically occur 5-14 days post-vaccination due to limited replication of the attenuated vaccine virus 4
- Not contagious: These vaccine-related symptoms represent a reaction to the attenuated virus, not true measles infection 4
- Diagnostic confusion: Positive measles tests within 14 days of vaccination likely represent vaccine virus detection rather than wild-type measles, given the extremely low incidence of wild-type measles 4
Household and Community Transmission Prevention
The vaccine also provides substantial protection against transmission:
- After one dose: 81% effective at preventing transmission to household contacts (RR 0.19,95% CI 0.04 to 0.89) 1
- After two doses: 85% effective at preventing transmission (RR 0.15,95% CI 0.03 to 0.75) 1
- After three doses: 96% effective at preventing transmission (RR 0.04,95% CI 0.01 to 0.23) 1
Immunity Gaps and Special Populations
Certain populations may have lower protection rates:
- Women of childbearing age: Studies show only 75% baseline measles seropositivity in some adult populations, increasing to 98% after MMR vaccination 5
- Inadequate childhood vaccination: Adults who received inadequate childhood vaccination or were vaccinated before 12 months of age may have suboptimal protection 3
- Inactivated vaccine recipients (1963-1967): Those who received killed measles vaccine require revaccination with two doses of live MMR vaccine 3
Clinical Bottom Line
The overwhelming majority (95-96%) of properly vaccinated individuals will not contract measles even when exposed. The small percentage who do contract measles after vaccination typically experience milder disease than unvaccinated individuals. The benefits of vaccination far outweigh the minimal risk of vaccine failure, particularly given that natural measles infection carries substantially higher risks of serious complications including encephalopathy (1 per 1,000 cases) compared to the vaccine (approximately 1 per 2 million doses) 3.