Millions of children are slipping through the cracks of global vaccination efforts — and measles, one of the deadliest and most contagious diseases, is being overlooked. New research calls for urgent action to put measles vaccination at the center of global health priorities.

Commentary: Missed Measles Immunisations Places Individuals and Communities at Risk: The Equity Argument for Including Measles in Under-Immunised Definitions. Image Credit: nobeastsofierce / Shutterstock

In a recent study published in the journal Vaccines, researchers in Australia, Papua New Guinea, and the United States estimated the burden of missed measles vaccination.

Measles is among the leading causes of mortality in children due to vaccine-preventable diseases. It also serves as an early warning sign or “canary in the coalmine” for weaknesses in health systems and immunization programs. Besides, it is highly transmissible and requires 95% vaccine coverage in the population for herd immunity compared to 90% for other vaccines. In 2019, more than 207,500 deaths occurred due to measles, which was 50% higher than in 2016.

Moreover, the increase in measles incidence and deaths has worsened due to reduced immunization coverage during the coronavirus disease 2019 (COVID-19) pandemic. Only 83% of children worldwide received the first measles-containing vaccine dose (MCV1) in 2022, whereas the global MCV1 coverage has remained 85%–86% since 2015. Importantly, this represents an additional 2.7 million children globally who missed out on MCV1 in 2022 compared to 2019.

According to Gavi, the vaccine alliance, zero-dose children are those lacking the first diphtheria-tetanus-pertussis (DTP)-containing vaccine dose (DTP1). In contrast, children who missed the third DTP dose (DTP3) are considered under-immunized. While these definitions are critical to highlighting priority groups, more work is required to understand whether children who missed DTP1 represent all of those who have missed routine immunizations, including the highly important measles vaccine.

Notably, recent modeling by the World Health Organization (WHO) shows that of the 154 million lives saved by immunization since 1974, over 60% are attributable to measles vaccination — more than for any other vaccine.

The researchers also emphasize that vaccine misinformation, such as the debunked claim linking measles vaccines to autism, continues to undermine public trust and vaccination uptake, posing a persistent challenge to improving measles coverage.

About the study

In the present study, researchers compared the number of under-immunized and zero-dose children to that of children missing measles vaccination to measure the additional burden of missed measles immunization. They used the World Health Organization (WHO)–United Nations International Children’s Emergency Fund (UNICEF) estimates of national immunization coverage data to analyze MCV1, DTP1, and DTP3 coverage in each country in 2019 and 2022.

The number of children who missed MCV1, DTP1, or DTP3 was computed by multiplying the inverse of immunization coverage with the number of children aged 12–23 months in each country. Further, dropout was calculated as the difference in the proportion of children who received one vaccine but did not receive the second one.

Dropout was estimated for three indicators in 2022: 1) difference in DTP1-to-MCV1 coverage, 2) DTP1-to-DTP3 coverage, and 3) MCV1-to-MCV2 coverage. Countries were ranked by the proportion and number of children who did not receive MCV1 and DTP1. Finally, the number of children missing immunizations was compared between different groups using Wilcoxon signed-rank tests.

Findings

In total, 21 million and 14 million children missed MCV1 and DTP1 globally in 2022, respectively. Of these, 96% lived in low- and middle-income countries (LMICs), underscoring that resource-limited settings face a disproportionate burden. Notably, the number of children who missed MCV1 was significantly higher than those who missed DTP1 or DTP3 in 2022.

The countries with the most children missing immunization were consistent across MCV1, DTP1, and DTP3: Afghanistan, Angola, India, Indonesia, Ethiopia, the Philippines, the Democratic Republic of Congo, Nigeria, Somalia, and Pakistan. In 2022, the median global coverage of MCV1 (90%) was significantly lower than that of DTP3 (91%) or DTP1 (95%).

Excluding high-income countries (HICs) from the analysis reduced the median MCV1 coverage to 83%. The median DTP1 to MCV1 dropout was two percentage points in HICs but six percentage points in LMICs. Moreover, there was a sustained dropout between MCV1 and MCV2 vaccinations in 2022. This highlights the challenge of sustaining engagement with health services for second-dose measles vaccines, especially in LMICs.

Countries with the highest proportions of children missing MCV1 and DTP1 were not reflective of those with the highest number of children missing these vaccinations.

For example, 56% and 55% of children in Papua New Guinea (PNG) missed MCV1 and DTP1 in 2022, respectively, but the number of children missing these vaccines was 137,263 and 134,812, respectively. Papua New Guinea also reports high rates of subacute sclerosing panencephalitis (SSPE), a severe and fatal complication following measles infection, demonstrating the grave consequences of insufficient vaccine coverage. In contrast, despite MCV1 coverage being 34% higher (than PNG), the number of children who did not receive MCV1 in the Philippines was approximately 752,000.

Further, using DTP1 coverage as a proxy for MCV1 coverage underestimated countries at the highest risk of measles outbreaks. That is, among the top 50 countries with the lowest coverage of MCV1 in 2022, 20% were not among the top 50 countries with the lowest coverage of DTP1. For instance, Montenegro had the second lowest coverage of MCV1 in 2022 but was absent from the list of countries with the lowest DTP1 coverage — highlighting gaps in measles-specific immunity that might be missed when focusing solely on DTP1.

Difference in diphtheria-tetanus-pertussis (DTP) and measles-containing vaccine (MCV) coverage in 20 countries with lowest MCV1 coverage, 2022.

Conclusions

Almost all children who missed the measles vaccine live in LMICs, and the dropout between DTP1 and MCV1 was three-fold higher in LMICs than in HICs. The authors posit that the absence of MCV1 vaccination should be considered in global definitions for under-immunized children, given the huge burden posed by measles outbreaks, high transmissibility, and lower MCV1 coverage (than DTP3).

Strategies for (more) equitable measles vaccination may include implementing measles microarray patches, which could decrease the need for cold chain and enable (trained) community members to administer vaccines, and transitioning from 10-dose to five-dose vaccine vials, which could help reduce healthcare workers’ reluctance to open new vials for a few vaccines.

The authors also caution that focusing zero-dose efforts on countries with the largest number of unvaccinated children may overlook smaller nations or fragile settings with very high proportions of missed vaccinations — populations equally vulnerable to outbreaks.

As a key metric in estimating and reporting under and unimmunized children worldwide, measles vaccination coverage should be explicitly included alongside DTP-based metrics. This is crucial to create awareness about the need to improve measles vaccination coverage and reduce the number of children who miss life-saving vaccinations. Given that measles accounts for the majority of life-years saved by vaccines since 1974, elevating MCV1 as a core global indicator would align global monitoring efforts with the reality of disease burden.