Susan E. Reef

Background: Vaccination has reduced the global incidence of measles to the lowest rates in history. However, local interruption of measles virus transmission requires sustained high levels of population immunity that can be challenging to achieve and maintain. The herd immunity threshold for measles is typically stipulated at 90–95%. This figure does not easily translate into age-specific immunity levels required to interrupt transmission. Previous estimates of such levels were based on speculative contact patterns based on historical data from high-income countries. The aim of this study was to determine age-specific immunity levels that would ensure elimination of measles when taking into account empirically observed contact patterns.Methods: We combined estimated immunity levels from serological data in 17 countries with studies of age-specific mixing patterns to derive contact-adjusted immunity levels. We then compared these to case data from the 10 years following the seroprevalence studies to establish a contact-adjusted immunity threshold for elimination. We lastly combined a range of hypothetical immunity profiles with contact data from a wide range of socioeconomic and demographic settings to determine whether they would be sufficient for elimination.Results: We found that contact-adjusted immunity levels were able to predict whether countries would experience outbreaks in the decade following the serological studies in about 70% of countries. The corresponding threshold level of contact-adjusted immunity was found to be 93%, corresponding to an average basic reproduction number of approximately 14. Testing different scenarios of immunity with this threshold level using contact studies from around the world, we found that 95% immunity would have to be achieved by the age of five and maintained across older age groups to guarantee elimination. This reflects a greater level of immunity required in 5–9-year-olds than established previously.Conclusions: The immunity levels we found necessary for measles elimination are higher than previous guidance. The importance of achieving high immunity levels in 5–9-year-olds presents both a challenge and an opportunity. While such high levels can be difficult to achieve, school entry provides an opportunity to ensure sufficient vaccination coverage. Combined with observations of contact patterns, further national and sub-national serological studies could serve to highlight key gaps in immunity that need to be filled in order to achieve national and regional measles elimination.

Rubella, usually a mild rash illness in children and adults, can cause serious consequences when a pregnant woman is infected, particularly in early pregnancy. These serious consequences include miscarriage, fetal death or an infant born with birth defects (i.e., congenital rubella syndrome (CRS)). The primary purpose for rubella vaccination is the prevention of congenital rubella infection including CRS. Since 1969, several rubella virus vaccines have been licensed for use; however, until the 1990s, use of rubella-containing vaccine (RCV) was limited primarily to developed countries. In 1996, it was estimated that 110,000 infants with CRS were born annually in developing countries. In 2000, the first World Health Organization rubella vaccine position paper was published to guide introduction of RCV in national childhood immunization schedules. From 1996 to 2009, the number of countries that introduced RCV into their national routine childhood immunization programs increased by 57% from 83 countries in 1996 to 130 countries in 2009. In addition, three of the six WHO regions established rubella control and CRS prevention goals: Region of the Americas and Europe rubella elimination by 2010 and 2015, respectively, and Western Pacific Region-accelerated rubella control and CRS prevention by 2015. Also, during this time period, the number of rubella cases reported decreased from 670,894 in 2000 to 121,344 in 2009. Rubella control and prevention of CRS can be accelerated by integrating with current global measles mortality reduction and regional elimination activities.

The availability of vaccines that contain both measles and rubella components allows for the elimination of both diseases. Although routine infant vaccination with rubella vaccine has had profound effects on the incidence of both acquired and congenital rubella, mass vaccination rapidly stops circulation of the virus and prevents paradoxical increases in susceptibility of women that might result from decreased exposure in childhood. Whereas routine rubella vaccination has eliminated the infection from many developed countries, mass vaccination has rapidly accomplished the same goal in Latin America and the Caribbean, and is being applied in other developing country areas.

The goal of eliminating indigenous rubella and congenital rubella syndrome (CRS) in the United States in the near future is now within reach, because

To describe clinical presentation and epidemiology of US infants with congenital rubella syndrome (CRS) and to identify missed opportunities for maternal vaccination, data from CRS cases reported to the National Congenital Rubella Syndrome Registry (NCRSR) from 1985 through 1996 were analyzed. Missed opportunities for maternal vaccination were defined as missed postpartum, premarital, and occupational opportunities, that is, times when rubella vaccination is recommended but was not given. From 1985 through 1996, 122 CRS cases were reported to the NCRSR. The most frequent CRS-related defect was congenital heart disease. Of the reported infants with CRS, 44% were Hispanic. Of 121 known missed opportunities for rubella vaccination among 94 mothers of infants with indigenous CRS, 98 (81%) were missed postpartum opportunities. CRS continues to occur in the United States. Hispanic infants have an increased risk of CRS. Missed opportunities for postpartum rubella vaccination were identified for 52% of indigenous CRS cases.