Worldwide trend in measles incidence from 1980 to 2016: A pooled analysis of evidence from 194 WHO Member StatesY Krishnamoorthy, M Sakthivel, SK Eliyas, G Surendran, G Sarveswaran
Department of Preventive and Social Medicine, Jawaharlal Institute of Postgraduate Medical Education and Research, Puducherry, Tamil Nadu, India
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/jpgm.JPGM_508_18
Source of Support: None, Conflict of Interest: None
Keywords: Communicable diseases, epidemiology, interrupted time series analysis, measles
Measles is a highly contagious disease associated with high rates of morbidity and mortality among children. The average number of infections reported to Centre for Disease Control and Prevention alone ranged from 3 to 4 million in the prevaccination era, and there were around 1.2 million deaths attributed to measles globally. It was after the introduction of the measles vaccine in 1963 that the burden of the disease started showing a declining trend globally.
There have been multiple global resolutions to address measles elimination, notably by the World Health Assembly in 1989 to decrease measles-related mortality and morbidity and the World Summit for Children in 1990 to increase measles vaccination coverage. In 2001, the World Health Organization (WHO) and the United Nations Children's Fund published a 5-year strategic plan to reduce measles mortality by half by 2005. In 2012, the Health Assembly has endorsed the Global Vaccine Action Plan, with an objective of eliminating measles in four out of the six WHO regions by the year 2015 and in five regions by 2020. Due to socioeconomic, cultural, geographical, and health disparities between the regions, the elimination targets are also different for them. The Health Assembly put forward the Global Action Plan with the objective of eliminating measles in four WHO regions by 2015 and in five regions by 2020. An additional Measles and Rubella Strategic Plan was launched by the M&R Initiative in the same year. Due to all these efforts, an 84% drop in the number of measles deaths between 2000 and 2016 is estimated, and proportion of children receiving measles vaccine through routine immunization has gone up to 85% globally over the same time period. The year 2016 was a milestone in public health with respect to measles, with the number of measles-related deaths falling below 100,000 for the first time globally.
The decline, though remarkable, has not been uniform across countries and regions. Although the incidence rates of measles as well as the number of deaths and complications have come down globally, there are wide disparities in the rates of fall across nations and continents due to various reasons like low vaccination coverage, poor surveillance, and sociopolitical unrest. Even within the same WHO region, the incidence rates are different over the past three decades. The topic of trends in measles incidence is of interest due to infrequent resurgence of this infection in different parts of the world. For example, outbreaks of measles in Europe in the current year illustrate the importance of predicting trends from previous data due to unforeseen events; in such situation, people tend to lose faith in vaccination.
The definition for measles elimination is the absence of endemic measles transmission in a defined geographical area (e.g., region or country) for ≥12 months in the presence of a well-performing surveillance system. In order to achieve this elimination target, vaccination coverage against measles should reach above 95% among under-5 children. The Americans managed to achieve measles elimination in 2002 and sustained it for 10 years, but the reestablishment of endemic measles in Brazil in 2014 has compromised their position. This is an example of the risk from importations in areas with low vaccine coverage. The other regions also continue to have pockets of outbreaks.
A long-term trend analysis of the achievements of the regions till date will give a better picture of the feasibility of the targets stated. To the best of our knowledge, we could not find any studies reporting the worldwide trend of measles except for the WHO Global Observatory data on measles incidence per year. Hence, the objective of the study was to find the trend of measles from 1980 to 2016 with respect to the WHO regions, the World Bank income groups, and at the global scale.
Study design and data sources
We have conducted a descriptive study (secondary data analysis) using the data on incidence of measles cases from WHO Global Health Observatory (GHO). The GHO data repository acts as the gateway for various health-related indicators and statistics for all the 194 WHO Member States. Data on the measles-related statistics were available as incident number of cases from 194 individual Member States. These data were reported in timely fashion with annual expected frequency from member state authorities. The definition of measles case used for the data collection was clinical, epidemiological, or laboratory-confirmed cases.
This data from individual Member States were classified based on WHO regions and World Bank income groups to assess the trend of measles over the period of 1980 to 2016. As the data we analyzed had open access, ethical approval from Institutional Review Board was not required.
Data of 194 Member States were classified based on WHO region and World Bank income groups to provide the global, region-wise, and income-group-wise estimates on trend of incidence of measles cases. All analyses were carried out using STATA 12 software. The outcome variable (incidence of measles cases) was a count outcome, and hence, Poisson regression model was used to determine the change over time in the incidence of measles cases with year as the exposure variable. As the output of the model showed that variance was greater than mean, we sought to use negative binomial regression model rather than the previous Poisson regression model. Since the incidence of measles cases was reported for each member state over a period of time, “independence of observations,” one of the assumptions to be satisfied in negative binomial regression, was violated. Hence, we ran the model using generalized estimating equation, which would adjust for dependence in observations and clustering effect.
We included data on incidence of measles during the period of 1980 to 2016 from 194 countries around the world. Data were classified based on WHO region and World Bank income groups to provide the global, region-wise, and income-group-wise trend estimates.
Globally, there was a significant decrease in the reported number of measles cases from 4211,431 in 1980 to 132,325 in 2016, which is depicted in [Figure 1]. This accounts for around 8% decline in the measles cases every year (IRR – 0.92). Decline in the incidence of measles cases over the years was further analyzed after adjusting for clustering with the WHO regions. The results showed that there was true declining trend (IRR – 0.90) over the years with around 10% decline every year. Modeling with World Bank income groups as cluster also showed true declining trend with decline rate of 9% (IRR – 0.91) in the incident number of measles cases from 1980 to 2016.
Trend in reported number of measles cases has decreased substantially in all the WHO regions. [Figure 2] depicts the trend in incidence of measles cases based on WHO regions from 1980 to 2016. Highest decline in the measles cases was seen in American region (98% decline since 1980) followed by Eastern Mediterranean region (94% decline since 1980) with least decline in African region (59% decline since 1980). Negative binomial regression on region-wise estimates showed that the American region had 86% lesser number of cases (IRR – 0.14, 95% CI: 0.08–0.24), Eastern Mediterranean had 81% lesser cases (IRR – 0.19, 95% CI: 0.11–0.33), Southeast Asian region had 70% lesser cases (IRR – 0.30, 95% CI: 0.17–0.51), European region had 49% lesser cases (IRR – 0.51, 95% CI: 0.29–0.88), and Western Pacific region had 43% (IRR – 0.57, 95% CI: 0.33–0.98) lesser number of incident measles cases when compared to African region.
[Figure 3] shows the trend of measles cases based on the World Bank income groups from 1980 to 2016. Higher income countries had the highest decline in the number of measles cases (97% decline since 1980), whereas the low-middle-income countries had the least decline (79% decline). Higher income countries had 65% lesser number of measles cases (IRR – 0.35, 95% CI: 0.22–0.55) and low-income countries had 42% lesser number of cases (IRR – 0.58, 95% CI: 0.36–0.92) when compared to lower and middle-income countries.
Measles has been more prevalent and persistent issue over the past three and half decades in the low- and middle-income countries when compared to other countries in the world. Our study showed that there has been a decline of about 8% in the reported number of measles cases every year globally. Coming to the regional level estimates, American and Eastern Mediterranean region showed maximum decline with least number of cases when compared to other WHO regions, while the African region showed least decline with maximum number of measles cases worldwide. However, all the regions showed a true declining trend over the past four decades. At the income group level, true declining trend was found in all the groups of countries with maximum decline and least number of measles cases in higher income countries. Lower middle-income countries had more number of incident measles cases and lesser decline when compared to the low-income countries.
Since India, a low- and middle-income country, contributes to maximum burden of measles,, there might be more number of cases in low-middle-income group when compared to low-income group. However, reporting from the lower income countries can be poor which could have attributed for lesser number of cases in those countries. Another reason could be the lack of laboratory facilities available in such countries for diagnosis. Highest decline in American and high-income countries can be attributed to the effective vaccination coverage and robust surveillance system in addition to the improved quality of life of the people.
The decline rate found in the current study would not be feasible enough to achieve global vaccine action plan target to eliminate measles from five WHO regions by 2020. First strategy to move toward this target is to achieve at least 95% population immunity in all the districts of each of the countries through inclusion in routine immunization schedule and special campaigns vaccinating the target age groups. However, to achieve this, political will and commitment is required for adequate allocation of resources and trained manpower. In addition, regular surveillance with prompt investigation of the suspected cases in all the countries of the world will help in stopping the spread of disease. An example for strong political commitment and adequate resource allocation for achieving the elimination target of 2020 is India, one of the high burden countries for measles contributing to more than half of the global deaths. India has adapted the strategic plan for Measles Elimination and Rubella and Congenital Rubella Syndrome Control in the Southeast Asia Region 2014–2020 in a phased manner since 2017. Measles Rubella Campaign was introduced in February 2017 to vaccinate all the children between 9 months and 15 years with an aim to achieve 95% population immunity. Another initiative was the introduction of case-based surveillance instead of current outbreak-based surveillance. Most of the countries have included second dose of measles vaccine, which has potential impact in reduction of cases and deaths related to measles, especially in high burden countries like India.
Major strength of the study is the attempt to make consistent and comparable trend estimates of measles cases across all the regions in the world over the past four decades. We have pooled the data and analyzed using the statistical model, which will account for geographical clustering features of the regions and countries. Current study also adds to the limited literature available regarding worldwide trend of measles incidence.
Measles is one of the highly contagious infections and each new case can increase the chances of causing an outbreak. The goal of global elimination is to be taken very seriously by all stakeholders across the globe due to the highly contagious nature of the disease and the constant risk of reestablishment. In order for it to be achievable, all the regions have to work toward elimination concurrently. Policy and practice gaps causing missed opportunities for measles immunization should be addressed. Periodic monitoring of indicators needs to be done at national, subnational, and regional level to know the progress and success of strategy implemented targeting measles. Hence, there is a need to develop a case-based surveillance system for measles and sustain it through regular feedback in all the countries. It is important to establish laboratory network accredited for measles diagnosis. Lessons learnt from small pox eradication and polio's progress toward eradication can be incorporated to achieve elimination of measles throughout the world.
Current study showed that there was disproportionate declining trend in the incidence of measles cases across the WHO regions and World Bank income group countries. However, estimates obtained in the study showed true declining trend in all the regions and countries. However, the decline rate found in the current study will not be sufficient to achieve the measles elimination target by 2020. Hence, effective implementation of strategies such as measles immunization coverage, case-based surveillance, outbreak preparedness, and laboratory network should be implemented in all the WHO Member States to eliminate the disease throughout the world.
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Conflicts of interest
There are no conflicts of interest.
[Figure 1], [Figure 2], [Figure 3]