From the Guidelines

The mortality rate for primary measles is approximately 1-2 deaths per 1,000 reported cases in developed countries, such as the United States 1.

The risk of death from measles or its complications is greater for infants, young children, and adults than for older children and adolescents 1.
In developing countries, the case-fatality rate can be as high as 25% 1.

Measles can be severe and is most frequently complicated by diarrhea, middle ear infection, or bronchopneumonia 1.
Encephalitis occurs in approximately one of every 1,000 reported cases, with survivors often having permanent brain damage and mental retardation 1.

From the Research

The mortality rate of primary measles is a significant concern, particularly in developing countries. According to the available evidence:

Measles is a highly contagious, potentially fatal disease, with mortality remaining high in developing countries (>5%) 2.
The disease mainly affects young children, with a mortality peak in the first three years of life 2.
Measles remains a major public health concern, causing over 100,000 deaths per year worldwide, with the most affected continents being Africa, South America, and Asia 2.
The global measles vaccination program has been successful in reducing measles-related disease and deaths worldwide, with a 73% decrease in deaths from 2000 to 2018 3.
However, despite the availability of a safe and effective vaccine, measles is still endemic in many countries and causes considerable morbidity and mortality, especially among children in resource-poor settings 4.

Several factors contribute to the mortality rate of primary measles, including:

Complications such as otitis media, laryngotracheobronchitis, pneumonia, stomatitis, and diarrhea, which can affect many organs 4.
Neurological complications, which are uncommon but serious, and can occur during or soon after the acute disease or months or even years later 4.
Malnutrition, with child wasting accounting for the most measles-related death cases, followed by child underweight, child stunting, and vitamin A deficiency 5.
Low socio-demographic index (SDI), which is associated with higher age-standardized rates and lower vaccine coverage rates 5.

There are significant regional variations in the mortality rate of primary measles, with:

Countries in Africa, South America, and Asia being the most affected 2.
Lower-middle-income countries accounting for 66% of measles cases 6.
Countries with stronger historical vaccination programs and higher country income having higher median ages for case patients 6.