From the Guidelines
The first dose of the measles vaccine is not very effective in a 6-month-old child, providing only temporary protection in special circumstances such as outbreaks or international travel. The standard measles vaccine (MMR or measles-mumps-rubella) is typically not recommended for routine administration at 6 months because maternal antibodies still present in the infant's system can interfere with the vaccine's effectiveness, resulting in poor immune response 1. When given at this young age, the vaccine provides limited protection against measles. However, in certain situations, measles vaccination of infants as young as 6 months may be undertaken as an outbreak control measure, with monovalent measles vaccine preferred, but MMR vaccine may be administered if the monovalent vaccine is not readily available 1.
- Key points to consider:
- Maternal antibodies can neutralize the vaccine virus before it stimulates a proper immune response
- Children vaccinated with measles or MMR vaccine before the first birthday should be revaccinated at age 12-15 months and again before entering school 1
- The reduced effectiveness at 6 months occurs because the baby's immune system is still developing
- It is essential to note that the vaccine's effectiveness improves significantly when administered at the recommended age of 12-15 months, providing 85-95% protection against measles.
From the Research
Effectiveness of the First Dose of Measles Vaccine in a 6-Month-Old
- The effectiveness of the first dose of the measles vaccine in a 6-month-old is a topic of interest due to the potential for early exposure to measles in certain regions or situations.
- According to a systematic review and meta-analysis 2, earlier age at first vaccination may decrease measles protection and immunogenicity after one dose and might still have an impact on vaccine failures after two doses of measles vaccine.
- The study found that pooled measles risk ratios ranged from 3.56 for MCV1 at <9 months to 0.48 for MCV1 at ≥15 months, both compared to 12-14 months, indicating that older age at MCV1 improves antibody response and measles protection among one-dose recipients.
- Another study 3 evaluated the impact of MCV1 given at <12 months vs. ≥12 months of age on long-term immunogenicity and vaccine effectiveness, finding that infants vaccinated with MCV1 <12 months of age showed an overall higher risk of measles compared to ≥12 months of age.
- The risk of measles decreased with increasing age at first vaccination, with those vaccinated with one dose ≥15 months at a lesser risk compared to 12-14 months or <12 months 3.
- However, measles seroconversion and seropositivity were not affected by age at first vaccination, but antibody levels were significantly lower in the MCV1 <12-month group 3.
Recommendations and Guidelines
- The Advisory Committee on Immunization Practices (ACIP) recommends 2 doses of MMR vaccine routinely for children, with the first dose administered at age 12 through 15 months and the second dose administered at age 4 through 6 years before school entry 4.
- For prevention of rubella, 1 dose of MMR vaccine is recommended for persons aged ≥12 months 4.
- The ACIP also recommends that infants aged birth to 6 months exposed to measles receive immune globulin administered intramuscularly (IGIM) as postexposure prophylaxis 4.