Epidemiology

Prevalence, Risk Factors and Disease Patterns

Measles exhibits notable seasonal patterns that vary with climate, human behavior and social dynamics. In temperate regions, measles incidence typically peaks during late winter and early spring, as one might expect with increased indoor social contact, especially school attendance. In tropical regions, measles outbreaks are more common during the dry season, which may be due to higher birth rates and local population density changes that occur with seasonal migration out of agricultural areas. Before the introduction of vaccines, large measles outbreaks occurred every 2-5 years; now, outbreaks occur irregularly and are less predictable. Unless an outbreak is quickly contained, it can expand extremely rapidly; the basic reproduction number (R₀; a measure of the average number of secondary cases arising from a typical case in a susceptible population) for measles is estimated to be 12-18, with some studies reporting an R₀ of >200.

Measles epidemiology varies widely between and even within countries. Before vaccination was introduced, the disease posed a significant burden worldwide, affecting nearly everyone by the age of 15; yearly global incidence was estimated at 135 million cases and over 6 million deaths (case-fatality ratios are influenced by vaccination status, age at the time of infection, nutritional health of patients, prevalence of HIV and the effectiveness of the healthcare system). In the United States, approximately 3-4 million people developed measles each year, roughly equivalent to the annual number of births. Despite the availability of a safe and effective vaccine for nearly 60 years, measles continues to cause significant morbidity and mortality, especially among children in resource-poor areas. Africa and East and South Asia have the highest age-standardized rates of measles. The World Health Organization (WHO) initiated the Global Vaccine Action Plan in 2012 with the goal of eliminating measles by 2020. However, none of the WHO regions has achieved this goal; alarmingly, the global incidence of measles increased by 556% from 2016 to 2019.

The incidence of measles fell significantly during the COVID-19 pandemic. Since vaccination programs and surveillance systems were disrupted during that time, it is unclear if this was due to a true decrease in cases resulting from the nonpharmacologic measures implemented to control the pandemic, or if cases were underdiagnosed and underreported, or a combination of both. In 2022, large or disruptive measles outbreaks occurred in 37 countries across four WHO regions, marking a 68% increase compared to the previous year. Of these outbreaks, 76% (28) were in the African Region, 16% (6) in the Eastern Mediterranean Region, 5% (2) in the South-East Asia Region and 3% (1) in the European Region. Overall, 43% (31 of 72) of countries that met the measles surveillance sensitivity target experienced large or disruptive outbreaks in 2022.

Importantly, while the United States achieved measles elimination status in 2000, localized cases and outbreaks continue to occur. Since 2000, the yearly number of measles cases has generally remained under 200 in the US, except in 2011 (220 cases), 2014 (667 cases) and 2019 (1,274 cases). Worryingly, in the first half of 2024, 159 cases of measles were reported across 23 jurisdictions in the United States (US). Of these, 66% were outbreak-related (i.e., part of a cluster of 3 or more related cases), with the vast majority (84%) involving unvaccinated individuals. These data underscore the importance of vaccination for maintaining the elimination status of measles, at least until the disease is eradicated globally – a task both biologically and technically achievable, if currently unrealistic given the suboptimal immunization rates in many regions. The tremendous impact of measles elimination in the US, achieved via vaccination, is also illustrated by considering a counterfactual – without vaccination, there would be an estimated 2.9 to 3.6 million measles cases every year in the US, with 2,330 fatal cases. Keeping vaccination coverage high is of paramount importance, as the example of measles in the United Kingdom (UK) shows. Vaccination in the UK began in 1968, and the number of cases rapidly declined in the decades that followed. Special vaccination campaigns in 1988 and 1994 (the latter organized as part of a response to an outbreak in 1994) further increased vaccination coverage, which reached 92% in 1996. However, vaccination coverage started to decline as a result of a 1998 paper wrongly linking the measles, mumps, and rubella (MMR) vaccine with autism (see Improving Vaccination Uptake), and reached a low of 80% in 2004, with a surge in cases. Fortunately, vaccination rates improved afterward and measles was eliminated in the UK for the first time in 2016, after vaccination coverage rates reached 95%. Unfortunately, measles transmission became continuous again in 2018 and consequently the country lost its elimination status. After regaining elimination status in 2021, the UK may again lose it in 2024 as measles cases surge again. These single-country dynamics illustrate the continuous importance of maintaining high vaccination coverage.