Protecting Young Infants From Measles

In this issue of Pediatrics by Science et al,¹  measles antibody levels were assessed in a cross-sectional sample of infants in Ontario, Canada, where endemic measles transmission has been eliminated since 1998. Antibody levels waned quickly, and all infants were considered susceptible to measles by age 6 months. The increased susceptibility to measles among younger infants in an elimination setting is not surprising and has been noted previously. Measles vaccine–induced maternal antibodies result in lower levels of passively acquired antibodies in infants.^2,3  However, as the authors state, this leaves infants potentially susceptible to measles until they are old enough to receive the vaccine. In light of increasing measles outbreaks during the past year reaching levels not recorded in the United States since 1992 and increased measles elsewhere,^4,5  coupled with the risk of severe illness in infants, there is increased concern regarding the protection of infants against measles.

Current recommendations from the Advisory Committee on Immunization Practices allow measles vaccination as early as 6 months for infants who plan to travel internationally, infants with ongoing risk for exposure during measles outbreaks, and as postexposure prophylaxis.⁶  For routine vaccination, the measles-mumps-rubella vaccine (MMR) is recommended at age 12 to 15 months, with vaccination occurring as soon as possible on reaching age 12 months. Since the vaccine was first licensed in 1963, the routine measles vaccine schedule has changed several times in response to changes in the measles epidemiology and understanding of immune response after vaccination. Measles vaccination was initially recommended at age 9 months because the disease was common, complications were greatest in infants, and data suggested that maternal antibodies were no longer present. The recommended age was increased to 12 months in 1965 and then 15 months in 1976, with additional data suggesting prolonged persistence of maternally derived antibodies and a higher rate of vaccine failures among those vaccinated between 12 and 15 months, respectively.^7,8  In 1989, a 2-dose schedule was implemented because of measles outbreaks among vaccinated school-aged children.⁹  Since 1994, MMR has been recommended for children aged 12 to 15 months and 4 to 6 years.

Is it time to consider another change to the routine measles vaccine schedule? Determining the appropriate schedule is challenging for policy makers. Ideally, the schedule should minimize the risk of measles and its complications and optimize vaccine-induced protection.⁷  Seroconversion after measles vaccination is influenced by existing maternal antibodies. Infants of mothers with vaccine-derived measles antibodies are likely to respond to vaccination at an earlier age.^3,10  However, even in the absence of maternal antibodies, young infants do not develop high levels of antibodies after measles vaccination.^11,12  Early vaccination may also alter response after revaccination, leading to lower levels of the antibody compared with children who are vaccinated for the first time during the second year of life.^13,14  Nevertheless, age and the presence of maternal antibodies do not appear to affect T-cell responses.¹³ 

Despite ongoing outbreaks, the epidemiology does not support lowering the age for measles vaccination in the United States. With the exception of 2014 and 2019, the incidence of measles in the United States has been extremely low (<1 per 1 million population) since 2001.¹⁵  Infants age <12 months represent <15% of measles case patients.^5,15  Measles incidence among infants is substantially higher in countries where measles is endemic. Additionally, >70% of case patients in the United States were unvaccinated.^5,15  Thus, the current recommendations in the United States are consistent with World Health Organization recommendations: the measles vaccine should be administered at age 12 months in countries with low rates of measles transmission.¹⁶ 

The study by Science et al¹  serves as a reminder that infants are susceptible to measles at a younger age. Infants have previously relied on passive maternal antibodies and community protection through high coverage of the 2-dose measles vaccine in the population. In the era of elimination, high coverage of 2 doses of MMR becomes essential to reduce the risk of exposure to infection and protect infants who cannot be vaccinated.¹⁷  This strategy is effective; most importations do not lead to outbreaks. When outbreaks do occur, most occur in communities where vaccine coverage is suboptimal.⁵  In recent years, the proportion of unvaccinated case patients generally increased over time as the proportion of imported cases declined. This trend suggests that measles transmission in the United States is primarily due to failure to vaccinate, which is likely due to increased vaccine hesitancy, unfortunately.¹⁵  From January 1 to October 1, 2019, there are >1200 measles cases in the United States.⁵  The significant number of cases and prolonged duration of transmission puts the elimination status of the United States at risk. Health care providers must work to maintain high levels of coverage with 2 doses of MMR among vaccine-eligible populations and minimize pockets of susceptibility to prevent transmission to infants and prevent reestablishment of endemic transmission. During outbreaks, an early vaccination schedule is warranted and recommended because of the increased risk of infection.⁶ 

Finally, the resurgence of measles due to importations from abroad highlight the importance of US support for efforts to reduce measles globally. This would not only protect populations around the globe from measles and its complications but also enhance our own domestic health security.¹⁸ 

MMR

measles-mumps-rubella vaccine