Despite decades of progress, declining vaccine coverage during COVID-19 threatens a resurgence of measles, especially in vulnerable regions. This global study urges renewed focus on equitable immunization to prevent future outbreaks.

Study: Global, regional, and national trends of measles burden and its vaccination coverage among children under five years old: an updated systematic analysis from the Global Burden of Disease study 2021. Image Credit: Design_Cells / Shutterstock

Did you know that over 4 million children under the age of five contracted measles in 2021 alone? Despite being preventable with vaccines, measles continues to affect millions globally. In a recent study published in the International Journal of Infectious Diseases, researchers from China explored global measles trends and vaccination coverage, especially in vulnerable, low-income regions. They also examined the impact of the coronavirus disease 2019 (COVID-19) pandemic on measles vaccination coverage.

Measles Vaccination

Measles remains one of the most contagious viral diseases affecting children, especially in areas with low vaccination coverage. It spreads through respiratory droplets and can cause serious complications such as pneumonia, brain inflammation, and death. Thanks to childhood vaccination programs, global measles cases have significantly declined over the last few decades.

However, measles outbreaks still occur frequently, particularly in low- and middle-income countries, where access to vaccines and healthcare is often limited. Moreover, persistent regional and socioeconomic disparities in immunization rates have left some populations at higher risk.

The study also noted that even some wealthier regions can experience measles outbreaks, sometimes linked to vaccine hesitancy or small outbreaks in populations where baseline measles rates are already very low. In such high-income regions, further reductions in measles can be challenging due to public distrust of vaccines or shifting health policies.

Little was known about how the pandemic affected measles surveillance and vaccine delivery worldwide. As the world focused on containing COVID-19, routine immunization services were disrupted, potentially reversing years of progress in measles control. Additionally, the pandemic’s non-pharmaceutical interventions—such as mask-wearing, social distancing, and improved hygiene—may have temporarily reduced measles transmission, possibly masking the immediate effects of declining vaccination rates. The study authors caution that as COVID-19 restrictions are lifted, there is a risk of delayed increases in measles cases due to diminished vaccine coverage.

About the Study

The present study analyzed global trends in measles among children under five, using data from the Global Burden of Disease 2021 study. The researchers focused on three key metrics: incidence rates, mortality rates, and disability-adjusted life years (DALYs), which combine years of life lost due to early death and years lived with disability. The analysis covered data between 1990 and 2021, with a particular focus on changes before and during the COVID-19 pandemic.

The study included data from 204 countries and territories accessed through the Global Health Data Exchange. To ensure accuracy, the researchers used high-quality data sources such as World Health Organization (WHO) case notifications and modeled estimates of vaccination coverage for the first and second doses (MCV1 and MCV2, respectively).

Adjustments were made for data from 2020 and 2021 to account for reduced disease surveillance and immunization efforts, reflecting disruptions caused by the COVID-19 pandemic. The researchers employed a spatiotemporal regression model to estimate vaccination coverage data for MCV1 and MCV2, taking into account uncertainties while incorporating predictors such as healthcare access, conflict, and vaccine supply issues.

Sociodemographic status was also evaluated using the Socio-Demographic Index (SDI), which combined income per capita, education levels, and fertility rates. Based on SDI, the countries were grouped into five categories, and the trends were analyzed accordingly.

The study also compared vaccination coverage estimates from different sources, such as the Global Burden of Disease (GBD) study and WHO/UNICEF. While the numbers were generally consistent, small methodological differences exist, with GBD using a model that integrates multiple data sources to correct for reporting biases.

Results

The study found that measles cases, deaths, and disease burden among children under five globally declined sharply between 1990 and 2021. These declines were especially significant in high-income regions, where rates dropped by more than 99%. However, by 2021, there were 4.1 million cases, 48,100 deaths, and 4.2 million DALYs.

Furthermore, regional disparities remained stark, with sub-Saharan Africa having the highest burden in 2021, particularly in the Western and Eastern parts of the region. In contrast, the Caribbean showed the greatest reductions. Although most regions saw continued declines even during the pandemic, East Asia experienced a surprising increase in deaths and DALYs between 2019 and 2021.

At the national level, Somalia had the highest measles incidence, death, and DALY rates in 2021. Other countries with high burdens included Mali and Guinea-Bissau. During the pandemic, countries such as Zambia, Vanuatu, and Kenya also reported a rise in cases, reversing previous improvements.

Vaccination coverage declined during the pandemic. MCV1 coverage fell in 75 countries, and MCV2 declined in 68. Some countries, such as Montenegro, Vanuatu, and Paraguay, experienced significant declines in vaccine coverage. This trend was especially concerning in regions already struggling with low coverage rates.

The study confirmed a strong negative correlation between vaccination coverage and measles burden—higher vaccination rates were linked to greater disease reductions. Socioeconomic factors also played a role, with improvements in SDI being associated with better measles outcomes in low SDI regions. Interestingly, the study found that in regions with medium to high SDI, increases in SDI could sometimes be associated with higher estimated annual percentage changes in mortality and DALY rates, a pattern possibly related to vaccine hesitancy or the statistical effect of small outbreaks in otherwise low-burden populations.

However, the researchers noted that the models used in the study may have oversimplified the relationship between vaccination and disease trends. Furthermore, limited post-pandemic data also reduced the ability to capture long-term effects. Additionally, incomplete surveillance in some countries may have led to an underestimation of measles cases. The authors also pointed out that the short timeframe of available post-pandemic data, and the use of certain statistical methods, may have led to wide confidence intervals and a need for caution in interpreting the statistical certainty of trend changes.

Conclusions

In summary, the study revealed that between 1990 and 2021, global measles rates in young children declined significantly; however, regional disparities and a decline in vaccine coverage increased the risk of a resurgence in measles incidence.

Furthermore, the COVID-19 pandemic disrupted routine immunization, especially in low-income regions, increasing the risk of outbreaks. While short-term declines in measles incidence may have been influenced by pandemic-related public health measures, the longer-term impact of reduced vaccination coverage could lead to a resurgence of measles as these measures are lifted. The findings emphasized the need to strengthen vaccination programs and address health inequities to prevent a resurgence of measles and achieve the global measles elimination goals.