Combining serological and contact data to derive target immunity levels for achieving and maintaining measles elimination.

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.

Progress Toward Measles Elimination - Western Pacific Region, 2013-2017.

In 2005, the Regional Committee for the World Health Organization (WHO) Western Pacific Region (WPR)* established a goal for measles elimination† by 2012 (1). To achieve this goal, the 37 WPR countries and areas implemented the recommended strategies in the WPR Plan of Action for Measles Elimination (2) and the Field Guidelines for Measles Elimination (3). The strategies include 1) achieving and maintaining ≥95% coverage with 2 doses of measles-containing vaccine (MCV) through routine immunization services and supplementary immunization activities (SIAs), when required; 2) conducting high-quality case-based measles surveillance, including timely and accurate testing of specimens to confirm or discard suspected cases and detect measles virus for genotyping and molecular analysis; and 3) establishing and maintaining measles outbreak preparedness to ensure rapid response and appropriate case management. This report updates the previous report (4) and describes progress toward measles elimination in WPR during 2013-2017. During 2013-2016, estimated regional coverage with the first MCV dose (MCV1) decreased from 97% to 96%, and coverage with the routine second MCV dose (MCV2) increased from 91% to 93%. Eighteen (50%) countries achieved ≥95% MCV1 coverage in 2016. Seven (39%) of 18 nationwide SIAs during 2013-2017 reported achieving ≥95% administrative coverage. After a record low of 5.9 cases per million population in 2012, measles incidence increased during 2013-2016 to a high of 68.9 in 2014, because of outbreaks in the Philippines and Vietnam, as well as increased incidence in China, and then declined to 5.2 in 2017. To achieve measles elimination in WPR, additional measures are needed to strengthen immunization programs to achieve high population immunity, maintain high-quality surveillance for rapid case detection and confirmation, and ensure outbreak preparedness and prompt response to contain outbreaks.

Progress Toward Measles Elimination - African Region, 2013-2016.

In 2011, the 46 World Health Organization (WHO) African Region (AFR) member states established a goal of measles elimination* by 2020, by achieving 1) ≥95% coverage of their target populations with the first dose of measles-containing vaccine (MCV1) at national and district levels; 2) ≥95% coverage with measles-containing vaccine (MCV) per district during supplemental immunization activities (SIAs); and 3) confirmed measles incidence of <1 case per 1 million population in all countries (1). Two key surveillance performance indicator targets include 1) investigating ≥2 cases of nonmeasles febrile rash illness per 100,000 population annually, and 2) obtaining a blood specimen from ≥1 suspected measles case in ≥80% of districts annually (2). This report updates the previous report (3) and describes progress toward measles elimination in AFR during 2013-2016. Estimated regional MCV1 coverage† increased from 71% in 2013 to 74% in 2015.§ Seven (15%) countries achieved ≥95% MCV1 coverage in 2015.¶ The number of countries providing a routine second MCV dose (MCV2) increased from 11 (24%) in 2013 to 23 (49%) in 2015. Forty-one (79%) of 52 SIAs** during 2013-2016 reported ≥95% coverage. Both surveillance targets were met in 19 (40%) countries in 2016. Confirmed measles incidence in AFR decreased from 76.3 per 1 million population to 27.9 during 2013-2016. To eliminate measles by 2020, AFR countries and partners need to 1) achieve ≥95% 2-dose MCV coverage through improved immunization services, including second dose (MCV2) introduction; 2) improve SIA quality by preparing 12-15 months in advance, and using readiness, intra-SIA, and post-SIA assessment tools; 3) fully implement elimination-standard surveillance††; 4) conduct annual district-level risk assessments; and 5) establish national committees and a regional commission for the verification of measles elimination.

Development of a District-Level Programmatic Assessment Tool for Risk of Measles Virus Transmission.

All six World Health Organization (WHO) regions have now set goals for measles elimination by or before 2020. To prioritize measles elimination efforts and use available resources efficiently, there is a need to identify at-risk areas that are offtrack from meeting performance targets and require strengthening of programmatic efforts. This article describes the development of a WHO measles programmatic risk assessment tool to be used for monitoring, guiding, and sustaining measles elimination efforts at the subnational level. We outline the tool development process; the tool specifications and requirements for data inputs; the framework of risk categories, indicators, and scoring; and the risk category assignment. Overall risk was assessed as a function of indicator scores that fall into four main categories: population immunity, surveillance quality, program performance, and threat assessment. On the basis of the overall score, the tool assigns each district a risk of either low, medium, high, or very high. The cut-off criteria for the risk assignment categories were based on the distribution of scores from all possible combinations of individual indicator cutoffs. The results may be used for advocacy to communicate risk to policymakers, mobilize resources for corrective actions, manage population immunity, and prioritize programmatic activities. Ongoing evaluation of indicators will be needed to evaluate programmatic performance and plan risk mitigation activities effectively. The availability of a comprehensive tool that can identify at-risk districts will enhance efforts to prioritize resources and implement strategies for achieving the Global Vaccine Action Plan goals for measles elimination.

Using Seroprevalence and Immunisation Coverage Data to Estimate the Global Burden of Congenital Rubella Syndrome, 1996-2010: A Systematic Review.

BACKGROUND: The burden of Congenital Rubella Syndrome (CRS) is typically underestimated in routine surveillance. Updated estimates are needed following the recent WHO position paper on rubella and recent GAVI initiatives, funding rubella vaccination in eligible countries. Previous estimates considered the year 1996 and only 78 (developing) countries. METHODS: We reviewed the literature to identify rubella seroprevalence studies conducted before countries introduced rubella-containing vaccination (RCV). These data and the estimated vaccination coverage in the routine schedule and mass campaigns were incorporated in mathematical models to estimate the CRS incidence in 1996 and 2000-2010 for each country, region and globally. RESULTS: The estimated CRS decreased in the three regions (Americas, Europe and Eastern Mediterranean) which had introduced widespread RCV by 2010, reaching <2 per 100,000 live births (the Americas and Europe) and 25 (95% CI 4-61) per 100,000 live births (the Eastern Mediterranean). The estimated incidence in 2010 ranged from 90 (95% CI: 46-195) in the Western Pacific, excluding China, to 116 (95% CI: 56-235) and 121 (95% CI: 31-238) per 100,000 live births in Africa and SE Asia respectively. Highest numbers of cases were predicted in Africa (39,000, 95% CI: 18,000-80,000) and SE Asia (49,000, 95% CI: 11,000-97,000). In 2010, 105,000 (95% CI: 54,000-158,000) CRS cases were estimated globally, compared to 119,000 (95% CI: 72,000-169,000) in 1996. CONCLUSIONS: Whilst falling dramatically in the Americas, Europe and the Eastern Mediterranean after vaccination, the estimated CRS incidence remains high elsewhere. Well-conducted seroprevalence studies can help to improve the reliability of these estimates and monitor the impact of rubella vaccination.

Progress toward measles preelimination--African Region, 2011-2012.

In 2008, the 46 member states of the World Health Organization (WHO) African Region (AFR) adopted a measles preelimination goal to reach by the end of 2012 with the following targets: 1) >98% reduction in estimated regional measles mortality compared with 2000, 2) annual measles incidence of fewer than five reported cases per million population nationally, 3) >90% national first dose of measles-containing vaccine (MCV1) coverage and >80% MCV1 coverage in all districts, and 4) >95% MCV coverage in all districts by supplementary immunization activities (SIAs). Surveillance performance objectives were to report two or more cases of nonmeasles febrile rash illness per 100,000 population, one or more suspected measles cases investigated with blood specimens in ≥80% of districts, and 100% completeness of surveillance reporting from all districts. This report updates previous reports and describes progress toward the measles preelimination goal during 2011-2012. In 2012, 13 (28%) member states had >90% MCV1 coverage, and three (7%) reported >90% MCV1 coverage nationally and >80% coverage in all districts. During 2011-2012, four (15%) of 27 SIAs with available information met the target of >95% coverage in all districts. In 2012, 16 of 43 (37%) member states met the incidence target of fewer than five cases per million, and 19 of 43 (44%) met both surveillance performance targets. In 2011, the WHO Regional Committee for AFR established a goal to achieve measles elimination by 2020. To achieve this goal, intensified efforts to identify and close population immunity gaps and improve surveillance quality are needed, as well as committed leadership and ownership of the measles elimination activities and mobilization of adequate resources to complement funding from global partners.